§A2 (correctness): BreathingExtractor weighted fusion was an un-normalized sum.
When `weights` was supplied shorter than n, supplied entries were used raw while
the missing tail defaulted to uniform 1/n -- two scales summed with no
renormalization, silently mis-scaling the breathing signal by a factor of
weights.len(). Extract to fuse_weighted_residuals() and normalize by
Sigma(effective weights), mirroring heartrate::compute_phase_coherence_signal.
Tests: partial_weights_are_renormalized_not_scale_mixed,
partial_weights_fusion_is_weighted_average (both fail on old code).
§A3 (stability): the IIR resonator pole radius r = 1 - bw/2 diverges when the
pole MAGNITUDE |r| >= 1 (i.e. bw >= 4: a very low fs relative to band width) --
NOT merely when r is negative, as the research report stated (a negative r with
|r| < 1 is still stable; the comments/tests are corrected accordingly). On
divergence the filter overflows to +/-inf within ~600 frames, NaN-poisons acf0,
and the extractor stalls permanently. Clamp r to [0, 0.9999] AND finite-guard
the filter output before the history push (defense-in-depth, mirrors ADR-154 §3).
Applied to both heartrate.rs and breathing.rs. Tests:
{heartrate,breathing}::low_sample_rate_filter_stays_finite (fs=0.5, 0.1-0.9 Hz
band, 600-frame unit step -> all-finite; both panic on old code).
These files also carry the §A1 VecDeque window conversion (bit-identical).
Co-Authored-By: claude-flow <ruv@ruv.net>
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