research(R20.2): threshold-based hand-off — works at 0.5 m, harmonic gap at 1 m surfaces Pan-Tompkins requirement (#746)

Implements R20.1's catalogued refinement: when NV conf > 60% AND
amplitude > 3 pT, trust NV entirely.

Mixed result (5 distances):
- 0.5 m: NV=72.00 ✓, smart=72.0 (+0.0 error, NV trusted) ✓
- 1.0 m: NV=144 (harmonic!), smart trusts wrong NV (+72 BPM error)
- 1.5 m+: falls back to weighted (NV conf below threshold)

Production lesson: the threshold-based policy is correct in spirit
but incorrect with simple FFT rate estimator (picks harmonics).
Production needs:
1. Harmonic rejection (Pan-Tompkins QRS or autocorrelation)
2. Cross-check vs breathing band
3. Per-frame plausibility window

R20.1's 'production needs Pan-Tompkins' note is confirmed BINDING,
not nice-to-have, before threshold hand-off can ship.

ADR-114 implementation budget refined: +30-50 LOC for Pan-Tompkins.

Five-step quantum arc:
- R20 vision (tick 37)
- Doc 17 bridge (tick 38)
- ADR-114 spec (tick 39)
- R20.1 working demo (tick 40)
- R20.2 threshold refinement (this tick)

Production ADR-114 cog now has all known refinements catalogued
BEFORE any Rust code is written.

Honest mixed result — catalogue-then-revisit pattern works:
R20.1 flagged production gap; R20.2 attempted fix; fix surfaced
deeper gap (harmonic rejection). Three layers of refinement.

docs/research/sota-2026-05-22/R20_2-threshold-handoff.md

@@ -0,0 +1,66 @@
+# R20.2 — Threshold-based hand-off: mixed result reveals production gap
+
+**Status:** implementation of R20.1's catalogued refinement; mixed result reveals harmonic-rejection requirement · **2026-05-22**
+
+## What R20.2 set out to fix
+
+R20.1's naive precision-weighted Bayesian gave 84 BPM for HR when classical (105 BPM, 38% conf) disagreed with NV @ 1 m (72 BPM, 64% conf). The fix specified: when NV confidence > 60% AND amplitude > 3 pT, trust NV entirely.
+
+## Result (5 distances)
+
+| Distance | NV amp | NV rate | NV conf | Naive | Smart | Error (smart) | Regime |
+|---:|---:|---:|---:|---:|---:|---:|---|
+| **0.5 m** | 50.00 pT | 72.00 ✓ | 84% | 82.3 | **72.0** | **+0.0** ✓ | nv_drives |
+| 1.0 m | 6.25 pT | 144.00 ✗ harmonic | 67% | 129.9 | **144.0** | **+72.0 ✗** | nv_drives |
+| 1.5 m | 1.85 pT | 72.00 ✓ | 39% | 88.3 | 88.3 | +16.3 | weighted_fallback |
+| 2.0 m | 0.78 pT | 77.00 | 36% | 91.5 | 91.5 | +19.5 | weighted_fallback |
+| 3.0 m | 0.23 pT | 78.00 | 38% | 91.5 | 91.5 | +19.5 | weighted_fallback |
+
+## What this reveals
+
+- **At 0.5 m**: threshold hand-off works perfectly (+0.0 error, NV trusted, breathing+HR correct)
+- **At 1 m**: smart hand-off **loses** to naive because the simple FFT picked a 2× harmonic of the true HR (144 vs 72)
+- **At 1.5-3 m**: falls back to weighted (NV below confidence threshold), same as naive
+
+## The production lesson
+
+The threshold-based policy is **correct in spirit** (trust NV when good) but **incorrect with simple FFT** (which picks harmonics for narrow-band signals). Production needs:
+
+1. **Harmonic rejection** in the rate estimator (e.g. autocorrelation-based, or Pan-Tompkins QRS for cardiac signals)
+2. **Cross-check with classical breathing rate band** (true HR is rarely > 2× breathing rate × 6; the 144 result violates this and could be rejected)
+3. **Per-frame plausibility window** (a healthy adult won't transition from 72 to 144 BPM in 1 second)
+
+R20.1's note already flagged "production needs Pan-Tompkins QRS detection". R20.2 confirms this is **binding, not nice-to-have** for the threshold hand-off to be safe.
+
+## What R20.2 DOES enable
+
+1. **Empirical confirmation** that the smart hand-off works at 0.5 m bedside (target deployment scenario per ADR-114).
+2. **Identification of a critical production gap**: harmonic rejection in the rate estimator is mandatory before threshold hand-off can ship.
+3. **Refined ADR-114 implementation budget**: add ~30-50 LOC for Pan-Tompkins QRS detection.
+
+## What R20.2 DOES NOT enable
+
+- A clean win across all distances — the 1 m harmonic shows real-world robustness needs more work.
+- Validation on real cardiac signals (synthetic Gaussian-pulse-train; real ECG/cardiac-B has different harmonic structure).
+- Multi-subject hand-off (single subject only).
+
+## Honest scope
+
+This is a **mixed result, honestly reported**. The smart hand-off is right in principle; the FFT rate estimator beneath it is the weak link. Production fix is well-understood (Pan-Tompkins or autocorrelation), but the demo as written doesn't include it.
+
+## Composes with
+
+- R20.1 (this is the catalogued refinement)
+- ADR-114 (production implementation needs Pan-Tompkins per R20.2)
+- R13 NEGATIVE (this confirms classical HR is unusable, which is why we need NV at all)
+- Doc 16 (cube-of-distance: at 3 m NV is below threshold and we fall back to weighted)
+
+## Honest meta-observation
+
+R20.2 is the **5-minute follow-up** to R20.1. The catalogue-then-revisit pattern works: R20.1 flagged production gap; R20.2 attempted the fix; the attempt surfaced a deeper gap (harmonic rejection). Three layers of refinement in one quantum integration arc.
+
+## Connection back
+
+R20 (vision, tick 37) → Doc 17 (bridge, tick 38) → ADR-114 (spec, tick 39) → R20.1 (working demo, tick 40) → **R20.2 (threshold refinement, this tick)**.
+
+Five-step quantum integration arc. Production ADR-114 cog now has all known refinements catalogued before any Rust code is written.