fix(vitals safety): non-finite CSI frame permanently froze breathing+HR via IIR-state poisoning (self-heal) + noise-never-Valid pin (#1079)

* fix(vitals): self-heal IIR filters after non-finite CSI frame (ADR-021/ADR-158 §A1)

The 2nd-order resonator bandpass_filter in BreathingExtractor and
HeartRateExtractor latches each output y[n] into the filter state
(y1/y2). A single non-finite amplitude residual from a corrupt CSI
frame produced a NaN output that was written into the state. The
existing extract() is_finite() guard dropped that one sample from the
history buffer but never sanitized the poisoned filter state, so every
subsequent output stayed NaN, was rejected too, and the sliding-window
history never refilled: breathing AND heart-rate extraction went
silently dead (returning None forever) until reset().

On the vitals alert path this is a safety-relevant denial of service —
one bad frame stops monitoring with no error surfaced. Same class as the
calibration NaN bug (ADR-154 §3) and the firmware vitals fixes
(#998/#996/#987): prior hardening guarded the history boundary but not
the filter-state boundary.

Fix: when bandpass_filter computes a non-finite output it resets the IIR
state to default and returns 0.0, so the resonator recovers on the next
clean frame (the 0.0 is still dropped by the caller's finite-check, so no
spurious sample enters history).

Also de-magic the safety-critical HR physiological plausibility band into
named HR_PLAUSIBLE_MIN_BPM/HR_PLAUSIBLE_MAX_BPM consts (value-identical
40/180 BPM).

Pinned by:
- breathing::tests::nan_frame_does_not_permanently_poison_filter (FAILS pre-fix)
- breathing::tests::inf_mid_stream_does_not_freeze_history (FAILS pre-fix)
- heartrate::tests::nan_frame_does_not_permanently_poison_filter (FAILS pre-fix)
- heartrate::tests::pure_noise_is_never_reported_valid (fabricated-vital negative)
- heartrate::tests::plausibility_band_constants_pinned (de-magic value pin)

wifi-densepose-vitals --no-default-features: 55->60 lib tests, 0 failed.
Workspace green (3370 passed, 0 failed). Python proof unchanged (vitals
off the deterministic proof's signal path).

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

* docs(vitals): record IIR NaN/inf self-heal fix (ADR-021, CHANGELOG)

Document the wifi-densepose-vitals filter-state poisoning fix in ADR-021
Implementation Notes (parallel to the firmware #998/#996/#987 robustness
class) and add a CHANGELOG [Unreleased] Fixed entry. Notes the confirmed
clean dimensions with evidence (flat -> None; noise -> low-confidence
Unreliable, never Valid; harmonic-rich breathing -> not a confident false
HR; out-of-band BPM clamped).

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

docs/adr/ADR-021-vital-sign-detection-rvdna-pipeline.md

@@ -1092,6 +1092,12 @@ Two robustness bugs were fixed in the on-device edge path (`firmware/esp32-csi-n
 
 Both are pinned by host-buildable C99 tests in `firmware/esp32-csi-node/test/test_vitals_count_presence.c` (`make run_vitals`). The exact thresholds are documented constants pending on-device calibration against ground truth.
 
+### 2026-06 — Rust `wifi-densepose-vitals`: IIR filter NaN/inf self-heal (ADR-158 §A1)
+
+A correctness/safety review of the Rust extraction crate found a real bug parallel to the firmware robustness class above. The 2nd-order resonator `bandpass_filter` in both `breathing.rs` and `heartrate.rs` latches each output `y[n]` into its filter state (`y1`/`y2`). A single non-finite amplitude residual from a corrupt CSI frame produced a NaN `output` that was written into the state; the existing `extract()` `is_finite()` guard dropped that one sample from the history buffer **but never sanitized the poisoned filter state**, so every later output stayed NaN, was rejected too, and the sliding-window history never refilled — breathing **and** heart-rate extraction went silently dead (returning `None` forever) until `reset()`. On the alert path this is a safety-relevant denial of service (one bad frame stops vitals monitoring with no error surfaced).
+
+Fix: when `bandpass_filter` computes a non-finite `output`, it resets the IIR state to default and returns `0.0`, so the resonator self-heals on the next clean frame (the `0.0` is still dropped by the caller's finite-check, so no spurious sample enters history). Same shape as the calibration NaN bug (ADR-154 §3) — the prior hardening guarded the *history boundary* but not the *filter-state boundary*. Pinned by `breathing::tests::nan_frame_does_not_permanently_poison_filter`, `breathing::tests::inf_mid_stream_does_not_freeze_history`, and `heartrate::tests::nan_frame_does_not_permanently_poison_filter` (all FAIL pre-fix, verified by reverting). The review also de-magicked the HR physiological plausibility band into named `HR_PLAUSIBLE_MIN_BPM`/`HR_PLAUSIBLE_MAX_BPM` consts (value-identical 40/180 BPM) and added a fabricated-vital negative (`pure_noise_is_never_reported_valid` — broadband noise never yields a clinically `Valid` HR; the extractor honestly returns low-confidence `Unreliable`). Clean dimensions confirmed with evidence: flat/silent input → `None`; pure noise → low-confidence `Unreliable`, never `Valid`; harmonic-rich breathing with no cardiac component → low-confidence, not a confident false HR; out-of-band BPM rejected by the plausibility clamp.
+
 ## References
 
 - Ramsauer et al. (2020). "Hopfield Networks is All You Need." ICLR 2021. (ModernHopfield formulation)