feat(adr-185): P4 benchmarks, examples, and README extras for the SOTA wheels

ADR-185 §4.2 pytest-benchmark micro-benchmarks + runnable examples +
README extras table for the aether/meridian/mat bindings.

- python/bench/test_bench_{aether,meridian,mat}.py — follow the existing
  test_bench_vitals.py pattern (skipped by default; --benchmark-only).
- python/examples/{reid_from_csi,cross_room_calibrate,mat_triage}.py —
  typed, runnable, mypy --strict clean.
- python/README.md — SOTA extras table + example links.

Measured on a RELEASE wheel (maturin develop --release --features sota),
reference machine per ADR-117 §10:
  AETHER embed()          mean ~150 us/window   (target <2 ms)   PASS
    batch scaling 1/8/64: 140 / 1091 / 8509 us  (linear, no O(n^2)) PASS
  MERIDIAN normalize()    mean ~2.2 us/frame     (target <200 us) PASS
  MERIDIAN encode()       mean ~6.9 us           (target <200 us) PASS
  MAT ingest+scan_once()  mean ~40 ms/256-frame  (< 500 ms interval) PASS

Acceptance self-verification (ADR-185 §6), all run just now:
  §6.1 default wheel 279 KB (<=5 MB); build_features has no p6-* feature  PASS
  §6.2 pytest tests/test_aether.py    9/9   PASS
  §6.3 pytest tests/test_meridian.py  13/13 PASS
  §6.4 pytest tests/test_mat.py       7/7   PASS
  §6.5 benchmarks meet all targets (above)                                PASS
  §6.6 parity harness: 3/3 SHA golden gates green (cargo test --features
       sota, 6/6); CI *wiring* as a release gate is out of python/ scope  PARTIAL
  §6.7 SOTA accuracy bars on labeled fixtures: NOT met (no labeled
       fixtures / trained models available; parity proves path-equality,
       not accuracy)                                                       OPEN
  §6.8 .pyi stubs present for all three; mypy --strict on the 3 examples   PASS
  §6.9 base wheel `import wifi_densepose.{aether,meridian,mat}` raises a
       clear ImportError naming the extra                                  PASS
  No regression: 76 pre-existing tests pass on the default wheel.

Status NOT flipped to Accepted: §6.7 (accuracy bars) is unmet, §6.6 CI
wiring is pending, and the per-extra wheel-size hoists (sensing-server /
train / mat leaf crates) remain follow-ups. docs/adr/ is owned by another
agent this session, so the ADR ledger edit is deferred to that owner.

python/bench/test_bench_meridian.py

@@ -0,0 +1,29 @@
+"""ADR-185 §4.2 — MERIDIAN micro-benchmarks.
+
+Targets (release build, ADR-027 §4.1/§4.3 ×2 headroom): `normalize()`
+< 200 µs/frame, `encode()` < 200 µs.
+
+Run with:
+    pytest python/bench/test_bench_meridian.py --benchmark-only
+
+Validated on a RELEASE wheel; a debug wheel will be several× slower.
+"""
+
+from __future__ import annotations
+
+from wifi_densepose import meridian as mer
+
+
+def test_normalize_per_frame(benchmark) -> None:
+    norm = mer.HardwareNormalizer()
+    amp = [10.0 + 0.05 * k for k in range(64)]
+    phase = [0.01 * k for k in range(64)]
+    out = benchmark(lambda: norm.normalize(amp, phase, mer.HardwareType.Esp32S3))
+    assert len(out.amplitude) == 56
+
+
+def test_geometry_encode(benchmark) -> None:
+    enc = mer.GeometryEncoder(mer.MeridianGeometryConfig())
+    aps = [[0.0, 0.0, 2.5], [5.0, 0.0, 2.5], [0.0, 4.0, 2.5]]
+    out = benchmark(lambda: enc.encode(aps))
+    assert len(out) == 64