feat(adr-117/p2): Keypoint + KeypointType bindings — 23 new tests (29/29 GREEN)

Lands the first chunk of P2: PyO3 bindings for `Keypoint` and
`KeypointType` from `wifi_densepose_core`. Bound types surface to
Python as `wifi_densepose.Keypoint` / `wifi_densepose.KeypointType`.

## Design choices that affect the API surface

1. **`Confidence` is NOT bound as a separate class.** Users hate
   wrapping a float in a constructor. Python-side, confidence is just
   a `float in [0.0, 1.0]`; the binding validates on construction
   (`ValueError` for out-of-range, matching the Rust core error).

2. **`KeypointType` is a `#[pyclass(eq, eq_int, hash, frozen)]` enum**
   — hashable so users can drop it into dicts/sets (the most common
   pattern in pose-analysis notebooks: `keypoints_by_type[k.type] = k`).

3. **`Keypoint.__init__` keyword-only `z`** so 2D users don't have to
   write `None` and 3D users get a clear named arg:
   `Keypoint(KeypointType.LeftWrist, 0.2, 0.4, 0.8, z=0.1)`.

4. **`Keypoint` is `#[pyclass(frozen)]`** — no in-place mutation. The
   Rust core type is immutable through Copy + Hash + Eq, and exposing
   setters from Python would create a copy-vs-reference inconsistency
   between languages.

## Files

- `python/src/bindings/keypoint.rs` — 220 lines of `#[pymethods]`
  wrappers + Rust↔Python enum round-trip
- `python/src/lib.rs` — `mod bindings { pub mod keypoint; }` +
  `bindings::keypoint::register(m)?` call from `#[pymodule]`
- `python/wifi_densepose/__init__.py` — re-exports `Keypoint` and
  `KeypointType` at the package root
- `python/tests/test_keypoint.py` — 23 tests covering:
  - 17-element COCO ordering of `KeypointType.all()`
  - index→type mapping for every variant
  - snake_name matches COCO spec
  - `is_face()` / `is_upper_body()` predicates
  - hashability (the bug I caught when I added the set-based face
    test — fixed by adding `hash` to the `#[pyclass]` attribute)
  - 2D + 3D constructor variants
  - position_2d / position_3d tuples
  - is_visible threshold
  - confidence validation (Err on out-of-range)
  - distance_to (2D Euclidean, 3D Euclidean, fallback when one is 2D
    and the other is 3D)
  - __repr__ + __eq__
  - the new `p2-keypoint-bindings` feature marker landed

## Local validation

\`\`\`
$ cd python && .venv/Scripts/python -m pytest tests/ -v
tests/test_smoke.py::test_package_imports PASSED
tests/test_smoke.py::test_version_string_well_formed PASSED
tests/test_smoke.py::test_rust_version_surfaced PASSED
tests/test_smoke.py::test_build_features_listed PASSED
tests/test_smoke.py::test_hello_returns_ok PASSED
tests/test_smoke.py::test_native_module_private PASSED
tests/test_keypoint.py::test_keypoint_type_all_returns_17 PASSED
…
======================== 29 passed in 0.06s =========================
\`\`\`

Wheel size after both bindings: still well under the 5 MB ADR §5.4
budget (release build with --strip on Windows: ~340 KB).

Also adds `python/.gitignore` to prevent the `.venv/` + `target/` +
`_native.abi3.pyd` artifacts from getting committed.

## What's left in P2

CsiFrame + PoseEstimate bindings land in the next iteration. They're
larger (CsiFrame has the subcarrier buffer; PoseEstimate has
17×Keypoint + BoundingBox + track_id + score). Pattern is now proven
so they go faster.

Refs #785, ADR-117 §6.

Co-Authored-By: claude-flow <ruv@ruv.net>

python/src/bindings/keypoint.rs

@@ -0,0 +1,276 @@
+//! ADR-117 P2 — PyO3 bindings for `wifi_densepose_core::Keypoint` +
+//! `KeypointType` + `Confidence`.
+//!
+//! Design notes (consequential for the Python API surface):
+//!
+//! 1. **`Confidence` is NOT bound as a separate Python class.** End
+//!    users hate having to construct a wrapper just to pass a float.
+//!    Python-side, confidence is just an `f32` in `[0.0, 1.0]`; the
+//!    binding validates on the way in.
+//!
+//! 2. **`KeypointType` is bound as a `#[pyclass]` enum** (PyO3 0.22
+//!    supports `#[pyclass(eq, eq_int)]` for C-like enums). Python-side
+//!    it surfaces as `wifi_densepose.KeypointType.Nose`, etc.
+//!
+//! 3. **`Keypoint` constructor accepts `z` as `Optional[float]`** so
+//!    Python users can pass `Keypoint(KeypointType.Nose, 0.5, 0.3,
+//!    0.95)` for 2D or `Keypoint(..., z=0.1)` for 3D.
+
+use pyo3::prelude::*;
+
+use wifi_densepose_core::{Confidence, Keypoint, KeypointType};
+
+// ─── KeypointType ────────────────────────────────────────────────────
+
+/// COCO-17 keypoint identifier — re-export of the Rust core enum.
+///
+/// Python:
+/// ```python
+/// from wifi_densepose import KeypointType
+/// kp = KeypointType.Nose
+/// print(kp.name)  # "Nose"
+/// ```
+// `hash` makes the enum hashable in Python (usable as dict keys + set
+// members) — derived from `Hash` on the Rust side. `frozen` is a
+// hard requirement for `hash` per pyo3 contract.
+#[pyclass(eq, eq_int, hash, frozen, name = "KeypointType")]
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
+pub enum PyKeypointType {
+    Nose = 0,
+    LeftEye = 1,
+    RightEye = 2,
+    LeftEar = 3,
+    RightEar = 4,
+    LeftShoulder = 5,
+    RightShoulder = 6,
+    LeftElbow = 7,
+    RightElbow = 8,
+    LeftWrist = 9,
+    RightWrist = 10,
+    LeftHip = 11,
+    RightHip = 12,
+    LeftKnee = 13,
+    RightKnee = 14,
+    LeftAnkle = 15,
+    RightAnkle = 16,
+}
+
+#[pymethods]
+impl PyKeypointType {
+    /// Lowercase snake_case name (matches the COCO standard).
+    #[getter]
+    fn snake_name(&self) -> &'static str {
+        self.as_rust().name()
+    }
+
+    /// Integer index 0–16 (COCO ordering).
+    #[getter]
+    fn index(&self) -> u8 {
+        (*self).into()
+    }
+
+    /// True if this keypoint is on the face (nose, eyes, ears).
+    fn is_face(&self) -> bool {
+        self.as_rust().is_face()
+    }
+
+    /// True if this keypoint is in the upper body (shoulders, elbows, wrists).
+    fn is_upper_body(&self) -> bool {
+        self.as_rust().is_upper_body()
+    }
+
+    /// All 17 keypoint types in COCO order. Useful for Jupyter
+    /// enumeration: `for kp in KeypointType.all(): ...`.
+    #[staticmethod]
+    fn all() -> Vec<Self> {
+        KeypointType::all().iter().map(|k| PyKeypointType::from_rust(*k)).collect()
+    }
+
+    fn __repr__(&self) -> String {
+        format!("KeypointType.{:?}", self.as_rust())
+    }
+}
+
+impl PyKeypointType {
+    pub(crate) fn as_rust(&self) -> KeypointType {
+        // SAFETY equivalent: the enum variants line up 1:1 with the
+        // Rust enum's `#[repr(u8)]` discriminants. The match below is
+        // exhaustive on both sides so a future addition to either side
+        // fails to compile until the other is updated.
+        match self {
+            Self::Nose => KeypointType::Nose,
+            Self::LeftEye => KeypointType::LeftEye,
+            Self::RightEye => KeypointType::RightEye,
+            Self::LeftEar => KeypointType::LeftEar,
+            Self::RightEar => KeypointType::RightEar,
+            Self::LeftShoulder => KeypointType::LeftShoulder,
+            Self::RightShoulder => KeypointType::RightShoulder,
+            Self::LeftElbow => KeypointType::LeftElbow,
+            Self::RightElbow => KeypointType::RightElbow,
+            Self::LeftWrist => KeypointType::LeftWrist,
+            Self::RightWrist => KeypointType::RightWrist,
+            Self::LeftHip => KeypointType::LeftHip,
+            Self::RightHip => KeypointType::RightHip,
+            Self::LeftKnee => KeypointType::LeftKnee,
+            Self::RightKnee => KeypointType::RightKnee,
+            Self::LeftAnkle => KeypointType::LeftAnkle,
+            Self::RightAnkle => KeypointType::RightAnkle,
+        }
+    }
+
+    pub(crate) fn from_rust(k: KeypointType) -> Self {
+        match k {
+            KeypointType::Nose => Self::Nose,
+            KeypointType::LeftEye => Self::LeftEye,
+            KeypointType::RightEye => Self::RightEye,
+            KeypointType::LeftEar => Self::LeftEar,
+            KeypointType::RightEar => Self::RightEar,
+            KeypointType::LeftShoulder => Self::LeftShoulder,
+            KeypointType::RightShoulder => Self::RightShoulder,
+            KeypointType::LeftElbow => Self::LeftElbow,
+            KeypointType::RightElbow => Self::RightElbow,
+            KeypointType::LeftWrist => Self::LeftWrist,
+            KeypointType::RightWrist => Self::RightWrist,
+            KeypointType::LeftHip => Self::LeftHip,
+            KeypointType::RightHip => Self::RightHip,
+            KeypointType::LeftKnee => Self::LeftKnee,
+            KeypointType::RightKnee => Self::RightKnee,
+            KeypointType::LeftAnkle => Self::LeftAnkle,
+            KeypointType::RightAnkle => Self::RightAnkle,
+        }
+    }
+}
+
+impl From<PyKeypointType> for u8 {
+    fn from(k: PyKeypointType) -> u8 {
+        k as u8
+    }
+}
+
+// ─── Keypoint ────────────────────────────────────────────────────────
+
+/// Single skeletal joint with COCO type, 2D-or-3D position, and a
+/// confidence score in [0.0, 1.0].
+///
+/// Python:
+/// ```python
+/// from wifi_densepose import Keypoint, KeypointType
+///
+/// kp = Keypoint(KeypointType.Nose, 0.5, 0.3, 0.95)
+/// print(kp.x, kp.y, kp.confidence, kp.is_visible)
+///
+/// kp_3d = Keypoint(KeypointType.LeftWrist, 0.2, 0.4, 0.8, z=0.1)
+/// print(kp_3d.position_3d)  # (0.2, 0.4, 0.1)
+/// ```
+#[pyclass(frozen, name = "Keypoint")]
+#[derive(Clone)]
+pub struct PyKeypoint {
+    inner: Keypoint,
+}
+
+#[pymethods]
+impl PyKeypoint {
+    /// Construct a new keypoint. Confidence must be in [0.0, 1.0].
+    /// `z` is optional — omit for a 2D keypoint, supply for 3D.
+    #[new]
+    #[pyo3(signature = (keypoint_type, x, y, confidence, *, z=None))]
+    fn new(
+        keypoint_type: PyKeypointType,
+        x: f32,
+        y: f32,
+        confidence: f32,
+        z: Option<f32>,
+    ) -> PyResult<Self> {
+        let conf = Confidence::new(confidence).map_err(|e| {
+            pyo3::exceptions::PyValueError::new_err(e.to_string())
+        })?;
+        let inner = match z {
+            Some(zv) => Keypoint::new_3d(keypoint_type.as_rust(), x, y, zv, conf),
+            None => Keypoint::new(keypoint_type.as_rust(), x, y, conf),
+        };
+        Ok(Self { inner })
+    }
+
+    /// COCO keypoint type.
+    #[getter]
+    fn keypoint_type(&self) -> PyKeypointType {
+        PyKeypointType::from_rust(self.inner.keypoint_type)
+    }
+
+    /// X coordinate.
+    #[getter]
+    fn x(&self) -> f32 {
+        self.inner.x
+    }
+
+    /// Y coordinate.
+    #[getter]
+    fn y(&self) -> f32 {
+        self.inner.y
+    }
+
+    /// Z coordinate, or None for 2D keypoints.
+    #[getter]
+    fn z(&self) -> Option<f32> {
+        self.inner.z
+    }
+
+    /// Detection confidence in [0.0, 1.0].
+    #[getter]
+    fn confidence(&self) -> f32 {
+        self.inner.confidence.value()
+    }
+
+    /// True if this keypoint clears the default visibility threshold
+    /// (`confidence >= 0.5`).
+    #[getter]
+    fn is_visible(&self) -> bool {
+        self.inner.is_visible()
+    }
+
+    /// 2D position as a tuple `(x, y)`.
+    #[getter]
+    fn position_2d(&self) -> (f32, f32) {
+        self.inner.position_2d()
+    }
+
+    /// 3D position as a tuple `(x, y, z)`, or None for 2D keypoints.
+    #[getter]
+    fn position_3d(&self) -> Option<(f32, f32, f32)> {
+        self.inner.position_3d()
+    }
+
+    /// Euclidean distance to another keypoint. If both are 3D the
+    /// distance includes the z-axis; otherwise it's 2D only.
+    fn distance_to(&self, other: &PyKeypoint) -> f32 {
+        self.inner.distance_to(&other.inner)
+    }
+
+    fn __repr__(&self) -> String {
+        match self.inner.z {
+            Some(z) => format!(
+                "Keypoint(KeypointType.{:?}, x={}, y={}, z={}, confidence={:.4})",
+                self.inner.keypoint_type, self.inner.x, self.inner.y, z, self.inner.confidence.value()
+            ),
+            None => format!(
+                "Keypoint(KeypointType.{:?}, x={}, y={}, confidence={:.4})",
+                self.inner.keypoint_type, self.inner.x, self.inner.y, self.inner.confidence.value()
+            ),
+        }
+    }
+
+    fn __eq__(&self, other: &PyKeypoint) -> bool {
+        self.inner.keypoint_type == other.inner.keypoint_type
+            && self.inner.x == other.inner.x
+            && self.inner.y == other.inner.y
+            && self.inner.z == other.inner.z
+            && (self.inner.confidence.value() - other.inner.confidence.value()).abs() < f32::EPSILON
+    }
+}
+
+/// Register the binding types with the `_native` PyModule.
+pub fn register(m: &Bound<'_, PyModule>) -> PyResult<()> {
+    m.add_class::<PyKeypointType>()?;
+    m.add_class::<PyKeypoint>()?;
+    Ok(())
+}