feat(cq): per-channel pulse-validity criterion (beat coverage >75%)

[boringethan]

Refs #126.

Adds a per-camera pulse-validity criterion to ContactQualityWorkflow: a channel that passes the existing signal-level checks is marked no_pulse unless a valid cardiac pulse train covers >75% of the CQ scan.

What's in it

• PulseWaveformAnalyzer.beat_coverage(total_duration_s, *, min_coverage=0.75) -> PulseCoverage — additive; reuses the Pulse-waveform analysis pipeline extension (BFI cardiac pulse: ensemble template, envelope, morphology) #124 band-limit + autocorrelation + foot segmentation, skips per-beat morphology QC (that QC is right for the live view but wrongly rejects beats on noisier channels for a presence gate). Noise rejection is delegated to the autocorrelation periodicity gate.
• CQ sink buffers per-(side,cam) bfi_live and runs one analyzer per channel at result(). Coverage denominator = global light-frame span (so a channel that drops out mid-scan scores low, not spuriously high).
• CamCQResult gains pulse_valid / pulse_coverage / pulse_hr_bpm / pulse_periodicity; reason gains no_pulse. Precedence: no_signal → ambient_light → poor_contact → no_pulse → ok.
• check(evaluate_pulse=False, pulse_min_coverage=0.75) — default off = fully backward-compatible (existing 1 s quick-check + all prior tests unchanged).

Criterion

pulse_valid = coverage > min_coverage AND periodicity >= 0.45

Validation

Bench capture scan_owC27EHALL (16 ch, ~85 bpm), vendored as tests/data/cq_pulse_reference_bfi.csv:

• all 16/16 known-good channels pass at 0.75 (coverage 0.90–0.94, periodicity 0.80–0.88)
• flat/noise references fail (periodicity 0.10/0.17)

Tests: 55 pass in the touched suite (pulse-coverage + live-pulse regression + CQ); 251 pass / 29 skipped in the pipeline suite. No hardware required.

Base

Targets feature/124 because it reuses the #124 PulseWaveformAnalyzer. Retarget to next if #124 merges first.

Scope / follow-up

SDK only. Bloodflow-app wiring (config keys, longer CQ capture, surface no_pulse in the CQ modal) is a separate follow-up ticket/PR.

Design: docs/superpowers/specs/2026-07-04-cq-pulse-validity-design.md

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