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/wifi_densepose/__init__.py

@@ -0,0 +1,66 @@
+"""WiFi-DensePose — passive human sensing from WiFi CSI.
+
+ADR-117 — v2.0 is a PyO3-bound replacement for the legacy pure-Python
+``wifi-densepose==1.1.0`` (released 2025-06-07). The compiled core is
+the same Rust workspace published in `v2/crates/` of the
+`ruvnet/RuView <https://github.com/ruvnet/RuView>`_ repository.
+
+Quick start::
+
+    import wifi_densepose
+    print(wifi_densepose.__version__)
+    print(wifi_densepose.__rust_version__)
+    print(wifi_densepose.hello())   # → "ok"
+
+P1 (this release): scaffold. Core types land in P2; vital signs +
+signal DSP in P3; WebSocket/MQTT client in P4. See the
+`ADR-117 modernization plan
+<https://github.com/ruvnet/RuView/blob/main/docs/adr/ADR-117-pip-wifi-densepose-modernization.md>`_
+for the full phase ledger.
+
+Migrating from v1.x: the v1 line was pure-Python and had a different
+API surface. v2 is a hard break (semver-justified). See the
+``v1.99.0`` tombstone wheel for the migration URL.
+"""
+
+from __future__ import annotations
+
+# Public Python version follows the wheel version, NOT the Rust core
+# version. The Rust core version is surfaced separately as
+# `__rust_version__` for diagnostics.
+__version__ = "2.0.0a1"
+
+# Re-export the compiled module's surface. The leading underscore on
+# `_native` is intentional — it marks the binding module as internal.
+# Users always import from `wifi_densepose` directly.
+from wifi_densepose import _native
+
+__rust_version__: str = _native.__rust_version__
+"""Version of the bound Rust core. Useful for bug reports."""
+
+__rust_build_tag__: str = _native.__rust_build_tag__
+"""Build tag of the Rust core (P5 will swap this for the git SHA)."""
+
+__build_features__: list[str] = list(_native.__build_features__)
+"""Feature flags the wheel was compiled with."""
+
+
+def hello() -> str:
+    """Smoke test — confirms the compiled module loads and is callable.
+
+    Returns:
+        Always ``"ok"`` if the wheel built and loaded correctly.
+
+    Used by ``python/tests/test_smoke.py`` to assert the PyO3 round-trip
+    works end-to-end on every cibuildwheel target.
+    """
+    return _native.hello()
+
+
+__all__ = [
+    "__version__",
+    "__rust_version__",
+    "__rust_build_tag__",
+    "__build_features__",
+    "hello",
+]