… 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>

…SIVE (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>

…nce + 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>