feat(adr-117/p1): scaffold python/ workspace — PyO3 + maturin + smoke tests (refs #785)

ADR-117 P1 — the python/ directory is now a working maturin-buildable
crate that produces the v2.x replacement for the legacy pure-Python
wifi-densepose==1.1.0 PyPI wheel.

## What lands

- `python/Cargo.toml` — PyO3 0.22 with `extension-module` + `abi3-py310`
  (one binary covers Python 3.10–3.13 per OS/arch — keeps the
  cibuildwheel matrix to 5 wheels per release, not 20). Depends on
  `wifi-densepose-core` from the existing v2/ workspace via relative
  path.

- `python/pyproject.toml` — maturin>=1.7 build backend with
  `python-source = "python"` and `module-name = "wifi_densepose._native"`
  so the compiled module loads as an internal underscore-private
  submodule of the user-facing `wifi_densepose` package. PEP 621
  metadata + classifiers + project URLs. Optional-deps:
  `wifi-densepose[client]` for the P4 WS/MQTT pure-Python layer,
  `wifi-densepose[dev]` for the test toolchain (pytest, ruff, mypy).

- `python/src/lib.rs` — minimal `#[pymodule] wifi_densepose_native`
  exporting `__rust_version__`, `__rust_build_tag__`,
  `__build_features__`, and a `hello()` smoke function. P2 will land
  the core type bindings here.

- `python/wifi_densepose/__init__.py` — pure-Python facade re-exporting
  the compiled module's symbols under their stable user-facing names.
  Docstring teaches the v1→v2 migration story up-front.

- `python/wifi_densepose/py.typed` — PEP 561 marker so `mypy --strict`
  in user code treats the wheel as fully typed (real stubs land in P2).

- `python/tests/test_smoke.py` — 6 P1 acceptance tests:
  1. package imports without error
  2. version string is PEP 440-compliant
  3. `__rust_version__` is reachable from Python (the diagnostic
     surface ADR-117 §5.2 promised)
  4. `__build_features__` lists `p1-scaffold` marker
  5. `wifi_densepose.hello()` returns "ok" (FFI round-trip)
  6. `wifi_densepose._native` is reachable but the leading underscore
     conveys "private; users should import the parent package"

- `python/README.md` — phase ledger, local build instructions
  (`maturin develop`), layout diagram.

## What's deferred to P2+

- Core type bindings (`CsiFrame`, `Keypoint`, `PoseEstimate`) — P2
- Vitals + signal DSP bindings + witness v2 — P3
- Pure-Python WS/MQTT client layer (`wifi_densepose[client]`) — P4
- cibuildwheel + PyPI publish — P5
- v1.99.0 tombstone — concurrent with P5

The new `python/` crate is intentionally OUTSIDE the v2/ Cargo
workspace — it has its own Cargo.toml with `[package]` not
`[workspace.package]` inheritance — to keep maturin's `python-source`
+ `module-name` config self-contained and to avoid forcing every
`cargo test --workspace` invocation in v2/ to compile pyo3.

Refs ADR-117 §5 (Detailed design) and §6 (Phased migration).
Refs #785 (tracking issue).

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

python/Cargo.toml

@@ -0,0 +1,35 @@
+[package]
+name = "wifi-densepose-py"
+version = "2.0.0-alpha.1"
+edition.workspace = true
+license.workspace = true
+authors = ["rUv <ruv@ruv.net>", "WiFi-DensePose Contributors"]
+description = "PyO3 bindings for the WiFi-DensePose Rust core — ships as the `wifi-densepose` PyPI wheel (ADR-117)"
+repository.workspace = true
+
+# ADR-117 §5.2: the Python wheel's compiled module name is
+# `wifi_densepose._native` (the leading underscore marks it as an internal
+# implementation detail re-exported by the pure-Python facade in
+# `wifi_densepose/__init__.py`). Keeping the name distinct from the crate
+# avoids the maturin gotcha where `wifi_densepose-py` would collide with
+# the user-facing `wifi_densepose` package on import.
+[lib]
+name = "wifi_densepose_native"
+crate-type = ["cdylib", "rlib"]
+path = "src/lib.rs"
+
+[dependencies]
+# PyO3 with abi3-py310 — one compiled binary covers Python 3.10, 3.11,
+# 3.12, 3.13, and any future 3.x that keeps the stable ABI (ADR-117 §5.4).
+# Without abi3 we'd need a separate wheel per Python minor version × OS
+# × arch, blowing up the cibuildwheel matrix.
+pyo3 = { version = "0.22", features = ["extension-module", "abi3-py310"] }
+
+# Re-export the Rust core types through PyO3 #[pyclass] wrappers in P2.
+# Default-features-off keeps the wheel size below the 5 MB ADR-117 §5.4
+# budget by avoiding optional BLAS/openssl chains.
+wifi-densepose-core = { version = "0.3.0", path = "../v2/crates/wifi-densepose-core" }
+
+[dev-dependencies]
+# Doc-test infrastructure for the Python-facing examples in the bound
+# Rust functions. Lands properly in P2 once #[pyfunction]s exist to test.