refactor(beyond-sota): ADR-155 M2 — host-verifiable §8 closeout (7 de-magic, 9 boundary tests, native-conv honest-null) (#1059)

* refactor(train): ADR-155 M2 §8 — de-magic train non-tch tuning constants + boundary tests

Lift bare numeric literals used as thresholds / guard epsilons in the
non-tch (host-verifiable) train surface into named, documented consts and
pin each set with a *_consts_unchanged_from_literals test. Values are
bit-identical to the prior inline literals — cleanup, no behaviour change.

De-magicked (const + pin test):
- metrics_core.rs: VISIBILITY_THRESHOLD (0.5), MIN_REFERENCE_EXTENT (1e-6),
  OKS_FALLBACK_SIGMA (0.07)
- ruview_metrics.rs: NUM_KEYPOINTS (17), VISIBILITY_THRESHOLD (0.5),
  PCK_THRESHOLD (0.2), MIN_BBOX_DIAG (1e-3), MIN_DURATION_MINUTES (1e-6)
- subcarrier.rs: SPARSE_BASIS_SIGMA (0.15), SPARSE_BASIS_THRESHOLD (1e-4),
  SPARSE_REGULARIZATION_LAMBDA (0.1), SPARSE_COO_PRUNE_EPS (1e-8),
  SPARSE_SOLVER_TOL (1e-5 f64), SPARSE_SOLVER_MAX_ITERS (500)
- eval.rs: MIN_POSITIVE_MPJPE (1e-10)
- domain.rs: LAYER_NORM_EPS (1e-5)
- virtual_aug.rs: BOX_MULLER_U1_FLOOR (1e-10), MIN_ROOM_SCALE (1e-10)

Boundary / characterization tests (pin CURRENT behaviour):
- visibility_threshold_boundary_is_inclusive (>= 0.5 at the edge)
- degenerate_extent_below_floor_is_unscoreable ((0,0,0.0)/0.0, not perfect)
- tracking_zero_duration_does_not_divide_by_zero
- oks_short_array_is_bounded_at_keypoint_count (16 rows, no panic)
- compute_interp_weights_single_target_is_index_zero (target_sc==1)
- sparse_interp_single_target_is_finite
- domain_gap_infinite_when_in_domain_perfect_but_cross_nonzero
- domain_gap_unity_when_everything_perfect
- augment_frame_zero_room_scale_passes_amplitude_finite

Doc-only (no behaviour change):
- rapid_adapt.rs: correct module-doc O(eps) -> O(eps^2) for central differences
- geometry.rs: add # Panics to DeepSets::encode (documents existing assert!)

train --no-default-features: 191 lib (was 176), 303 total (was 288), 0 failed.

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

* feat(nn): ADR-155 M2 §3 — pure-Rust LinearHead::try_new input guard + de-magic softplus threshold

ADR-155 §3 found rf_encoder.rs has no adversarial checkpoint-deserialization
assert — its assert_eq!s in LinearHead::new are construction-time API contracts
on programmer-supplied vectors. This adds the honest, in-scope improvement the
M2 task allows: a pure-Rust *fallible* constructor so weights from an untrusted /
deserialized checkpoint can be shape-validated without panicking.

- Add RfHeadError (WeightShape / BiasShape / VarWeightShape) + Display + Error.
- Add LinearHead::try_new returning Result<Self, RfHeadError>; on success the
  head is byte-identical to LinearHead::new. new() is unchanged (still asserts;
  now documents # Panics and points to try_new) — no behaviour change for
  existing callers.
- De-magic softplus's bare 20.0 overflow threshold into
  SOFTPLUS_LINEAR_THRESHOLD (value unchanged) + pin test.

Tests: try_new_accepts_valid_and_rejects_each_bad_shape (valid == new forward;
each bad shape → typed error), softplus_threshold_unchanged_from_literal.

nn --no-default-features lib: 37 passed (was 35), 0 failed.

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

* perf(nn): ADR-155 M2 §4 — native-conv bench-first → MEASURED-INCONCLUSIVE (no perf change shipped)

The §8 "native-conv naive-loop rewrite" backlog item: DensePoseHead::
apply_conv_layer is a pure-Rust 6-nested-loop conv (benchable on this host, not
tch/ort-gated). Bench-first per the §0 PROOF discipline.

- Add committed criterion bench benches/native_conv_bench.rs measuring forward()
  through the naive conv on representative single-layer configs (--no-default-
  features; no ort download).
- Prototyped a bit-identical range-clamped variant (hoist the per-tap in-bounds
  branch by pre-clamping kh/kw ranges; same ic→kh→kw MAC order ⇒ bit-identical).
  MEASURED before/after on this host: ~35% faster on padding-heavy small-channel
  maps (4.40→2.84 ms) but a ~3% *regression* on channel-heavy maps (11.09→11.48
  ms), all inside a ±20% run-to-run noise floor. Verdict: INCONCLUSIVE — the
  benefit is not robustly positive, so the rewrite is NOT shipped and NOT a
  fabricated speedup. Reverted to the naive loop; honestly deferred (ADR-155 §8).
- Add native_conv_matches_reference: a hand-computed characterization anchor
  (1×1 = scalar MAC; same-padded 3×3 ones = truncated-window sums 9/6/4) pinning
  CURRENT conv behaviour for any future rewrite.

nn --no-default-features lib: 38 passed (was 37), 0 failed. No behaviour change.

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

* docs(adr-155): M2 §8.2 — enumerated host-verifiable P3 backlog clearance + CHANGELOG

Replace the §8 bulk "~40 lower-severity findings" line with the real, enumerated
M2 resolution (§8.2): 7 de-magicked (const + pin == prior literal), 9 boundary
tests, 1 input guard (rf_encoder try_new), 2 doc-only, 1 perf bench-first
MEASURED-INCONCLUSIVE (not shipped). Mark native-conv + rf_encoder RESOLVED;
state which §8 items stay data-gated (GraphPose-Fi/INT4/CSI-JEPA) or tch-gated
(proof/trainer/model panic sites, metrics *_v2 dead code) and ONNX read-lock
upstream-gated — blocked, not dropped. Declare the non-tch-verifiable subset of
§8 cleared.

Validation: train --no-default-features 303 passed (was 288); nn lib 38 (was 35);
workspace --no-default-features 3,293 passed, 0 failed; Python proof VERDICT PASS,
hash f8e76f21…46f7a UNCHANGED bit-exact.

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

v2/crates/wifi-densepose-train/src/virtual_aug.rs

@@ -17,6 +17,15 @@
 
 use std::f32::consts::PI;
 
+/// Floor on the Box-Muller `u1` sample so `ln(u1)` stays finite when the PRNG
+/// returns ≈0 (ADR-155 M2 §8: de-magicked from a bare `1e-10`; value unchanged).
+const BOX_MULLER_U1_FLOOR: f32 = 1e-10;
+
+/// Magnitude below which `room_scale` is treated as zero and the amplitude
+/// division is skipped (guards `val / room_scale` against ÷≈0). De-magicked from
+/// a bare `1e-10`; value unchanged, no behaviour change.
+const MIN_ROOM_SCALE: f32 = 1e-10;
+
 // ---------------------------------------------------------------------------
 // Xorshift64 PRNG (matches dataset.rs pattern)
 // ---------------------------------------------------------------------------
@@ -67,7 +76,7 @@ impl Xorshift64 {
     /// Sample an approximate Gaussian (mean=0, std=1) via Box-Muller.
     #[inline]
     pub fn next_gaussian(&mut self) -> f32 {
-        let u1 = self.next_f32().max(1e-10);
+        let u1 = self.next_f32().max(BOX_MULLER_U1_FLOOR);
         let u2 = self.next_f32();
         (-2.0 * u1.ln()).sqrt() * (2.0 * PI * u2).cos()
     }
@@ -158,7 +167,7 @@ impl VirtualDomainAugmentor {
         for (k, &val) in frame.iter().enumerate() {
             let k_f = k as f32;
             // 1. Room-scale amplitude attenuation (guard against zero scale)
-            let scaled = if domain.room_scale.abs() < 1e-10 {
+            let scaled = if domain.room_scale.abs() < MIN_ROOM_SCALE {
                 val
             } else {
                 val / domain.room_scale
@@ -207,6 +216,42 @@ impl VirtualDomainAugmentor {
 mod tests {
     use super::*;
 
+    /// ADR-155 M2 §8: the de-magicked guard epsilons must equal the prior inline
+    /// `1e-10` literals exactly (operating-value guard).
+    #[test]
+    fn virtual_aug_guard_consts_unchanged_from_literals() {
+        assert_eq!(BOX_MULLER_U1_FLOOR, 1e-10_f32);
+        assert_eq!(MIN_ROOM_SCALE, 1e-10_f32);
+    }
+
+    /// Characterize the zero-room-scale guard: a `room_scale` of exactly 0 must
+    /// pass amplitude through unscaled (the guard branch), never produce
+    /// Inf/NaN from `val / 0`.
+    #[test]
+    fn augment_frame_zero_room_scale_passes_amplitude_finite() {
+        let aug = VirtualDomainAugmentor::default();
+        let domain = VirtualDomain {
+            room_scale: 0.0,
+            // reflection_coeff = 1.0 ⇒ refl = 1.0 + (1-1)·cos(..) = 1.0 (constant,
+            // so the reflection step is the identity for this characterization).
+            reflection_coeff: 1.0,
+            n_scatterers: 0, // no scatterer interference
+            noise_std: 0.0,  // no additive noise
+            domain_id: 1,
+        };
+        let frame = vec![1.0_f32, 2.0, 3.0, 4.0];
+        let out = aug.augment_frame(&frame, &domain);
+        assert_eq!(out.len(), frame.len());
+        assert!(
+            out.iter().all(|v| v.is_finite()),
+            "zero room_scale must not yield Inf/NaN: {out:?}"
+        );
+        // With every other transform neutralised, the guard leaves amplitude as-is.
+        for (o, f) in out.iter().zip(frame.iter()) {
+            assert!((o - f).abs() < 1e-6, "expected pass-through, got {o} vs {f}");
+        }
+    }
+
     fn make_domain(scale: f32, coeff: f32, scatter: usize, noise: f32, id: u32) -> VirtualDomain {
         VirtualDomain {
             room_scale: scale,