feat(adr-185): P2 MERIDIAN bindings (wifi_densepose.meridian) + parity harness

Bind the ADR-027 MERIDIAN cross-environment domain-generalization surface
into the wheel behind a gated [meridian] extra / Cargo `meridian` feature.
Inference/adaptation path only (tch-free), per ADR-185 section 3.3.

Surface (bound against the REAL code at HEAD, not the ADR wishlist):
- HardwareType / HardwareNormalizer / CanonicalCsiFrame (from
  wifi-densepose-signal::hardware_norm)
- MeridianGeometryConfig / GeometryEncoder (64-dim, permutation-invariant)
- RapidAdaptation / AdaptationResult (push_frame + adapt, LoRA deltas)
- CrossDomainEvaluator + mpjpe (from wifi-densepose-train, NO tch-backend)
All compute paths GIL-released (py.allow_threads).

Honest deviations from ADR section 3.3 (documented in the module header):
- ADR's RapidAdaptation.calibrate(windows) and AdaptationResult.converged
  DO NOT EXIST. Real API is push_frame + adapt(); result carries
  {lora_weights, final_loss, frames_used, adaptation_epochs}. Bound as-is.
- ADR's HardwareType.detect exposed as a staticmethod delegating to the
  real HardwareNormalizer::detect_hardware.
- ADR's normalize(frame: CsiFrame, hw) is really normalize(amplitude,
  phase, hw) over f64 vectors returning Result; bound faithfully.
- CanonicalCsiFrame fields are singular amplitude/phase (ADR said plural).
Training-time types (DomainFactorizer, GradientReversalLayer,
VirtualDomainAugmentor) are out of P6 scope (need the libtorch tier).

Parity (section 4.1, release-blocking): committed fixture
meridian_input.json -> native Rust reference (tests/meridian_parity.rs,
calls hardware_norm + geometry + rapid_adapt directly) locks
tests/golden/meridian_output.sha256 over the concatenated f32 outputs
(esp32+intel canonical frames, 64-dim geometry vector, rapid-adapt LoRA
weights); pytest (tests/test_meridian.py) runs the same fixture through
the binding and asserts the identical SHA-256. Both pass.

Verified:
  cargo test --features meridian --test meridian_parity -> 2/2 pass
  maturin develop --features meridian + pytest tests/test_meridian.py
    -> 13/13 pass
  default cargo build clean, 0 train/signal/sensing-server refs in the
    default dep graph (gate keeps the base wheel lean).

WHEEL-SIZE FINDING (ADR-185 section 9 / section 1.2): the libtorch risk
the ADR feared is AVOIDED -- wifi-densepose-train's `tch` dep is properly
optional (feature tch-backend, OFF), so no libtorch links. BUT train
still carries NON-optional deps: tokio (rt subset), the five ruvector-*
crates, and wifi-densepose-nn (which itself pulls `ort` / ONNX Runtime +
reqwest/hyper). So a [meridian] wheel exceeds the ADR-117 section 5.4
<=5 MB budget (though lighter than AETHER's axum/tokio server tree). The
clean fix is the same leaf-crate hoist: move the pure inference modules
(geometry, rapid_adapt, eval, hardware_norm) into a tch/tokio/ort-free
leaf crate. A required pre-release follow-up, not a functional blocker;
P2 binds real code and proves parity today.

python/src/lib.rs

@@ -20,6 +20,8 @@ mod bindings {
     #[cfg(feature = "aether")]
     pub mod aether;
     pub mod bfld;
+    #[cfg(feature = "meridian")]
+    pub mod meridian;
     pub mod keypoint;
     pub mod pose;
     pub mod privacy_gate;
@@ -47,6 +49,8 @@ fn build_features() -> Vec<&'static str> {
     feats.push("p3.5-bfld-bindings"); // BfldFrame + BfldReport + BfldKind (stub Rust)
     #[cfg(feature = "aether")]
     feats.push("p6-aether-bindings"); // ADR-185 P1 — AETHER contrastive embeddings
+    #[cfg(feature = "meridian")]
+    feats.push("p6-meridian-bindings"); // ADR-185 P2 — MERIDIAN domain generalization
     feats
 }
 
@@ -96,5 +100,11 @@ fn wifi_densepose_native(m: &Bound<'_, PyModule>) -> PyResult<()> {
     #[cfg(feature = "aether")]
     bindings::aether::register(m)?;
 
+    // ADR-185 P2 — MERIDIAN cross-environment domain-generalization
+    // bindings (hardware normalization, geometry encoding, rapid
+    // adaptation, cross-domain eval). Gated behind `meridian`; tch-free.
+    #[cfg(feature = "meridian")]
+    bindings::meridian::register(m)?;
+
     Ok(())
 }