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/README.md

@@ -0,0 +1,68 @@
+# `wifi-densepose` v2.x — PyO3 bindings for the Rust core
+
+This directory contains the source for the `wifi-densepose` PyPI wheel
+(v2.0+). It's a PyO3 + maturin build that wraps the Rust crates in
+[`v2/crates/`](../v2/crates/) and replaces the legacy pure-Python
+`wifi-densepose==1.1.0` (released 2025-06-07).
+
+See [ADR-117](../docs/adr/ADR-117-pip-wifi-densepose-modernization.md)
+for the full modernization plan.
+
+## Build locally
+
+```bash
+# Install maturin + dev deps
+pip install maturin pytest
+
+# Develop-install — builds the Rust extension in-place
+cd python
+maturin develop
+
+# Run the smoke tests
+pytest tests/
+```
+
+The `maturin develop` command produces a debug-build wheel installed
+into your current Python environment. For release builds:
+
+```bash
+maturin build --release --strip
+```
+
+The wheel lands under `python/target/wheels/`.
+
+## Layout
+
+```
+python/
+├── Cargo.toml                    # PyO3 + abi3-py310 + Rust deps
+├── pyproject.toml                # maturin backend + Python metadata
+├── README.md                     # this file
+├── src/
+│   └── lib.rs                    # #[pymodule] — Rust binding glue
+├── wifi_densepose/               # pure-Python facade (the user-facing API)
+│   ├── __init__.py               # re-exports compiled module symbols
+│   └── py.typed                  # PEP 561 typed-package marker
+└── tests/
+    └── test_smoke.py             # P1 acceptance tests
+```
+
+## Phase status (per ADR-117 §6)
+
+- ✅ **P1 — Scaffold (this commit)**: module loads, version constant
+  exposed, 6 smoke tests pass via `maturin develop`.
+- ⏳ **P2 — Core type bindings**: `CsiFrame`, `Keypoint`, `PoseEstimate`.
+- ⏳ **P3 — Vitals + signal DSP**: 4-stage HR/BR pipeline + `CsiProcessor`
+  + `PhaseSanitizer`, with `allow_threads` GIL release on hot loops.
+- ⏳ **P4 — WS/MQTT client**: pure-Python `wifi_densepose.client` extra.
+- ⏳ **P5 — cibuildwheel + PyPI publish**: Linux/macOS/Windows × abi3-py310.
+
+Each phase ends with a checkbox PR. Tests are additive — every phase's
+smoke tests must still pass after later phases land.
+
+## Migrating from v1.x
+
+The v1 line was a separate pure-Python implementation. v2 is a hard
+break (semver-justified by 11.5 months of stack drift). Migration
+guide ships in [docs/migrations/wifi-densepose-1-to-2.md](../docs/migrations/wifi-densepose-1-to-2.md)
+(landing in P5).