P1: Per-day reuse cache serves HRV-less night scans permanently when R-R lands after HR; force refresh does not bypass it

[bhelm]

Found by an independent review of the integrated fork state (2026-08-18). The code arrived with
the upstream range d4b7cc8..f2271cd (ryanbr#1005 / ryanbr#1395 / ryanbr#1396 per-day reuse cache); affects
upstream equally. Filed here first — forwarding upstream is a separate decision.

Problem

The per-day reuse cache in Strand/Data/IntelligenceEngine.swift:780 (same design in the
Kotlin twin, IntelligenceEngine.kt:617-643) keys a day's scan on owner + HR fingerprint
(count, maxTs) + skin anchor + config signature only. R-R / respiratory / SpO2 rows arriving
with no HR-row delta produce the identical key, so a stale cached DayScan is served.
Force-refresh does not bypass the day cache — force=true only bypasses the whole-pass
watermark gate.

Failure scenario

WHOOP history offload delivers channels independently, and offloaded HR rows duplicating live
rows are dropped by ON CONFLICT DO NOTHING. A banked night whose R-R packets land after its
HR is scored once with rmssd=nil and cached; every subsequent pass — including user-initiated
force refreshes — recomputes the same key and re-serves the HRV-less scan. Night HRV /
respiratory rate stay blank until an unrelated HR row, a config change, or an app restart
changes the key.

Possible directions

Fold the other stream fingerprints (R-R / resp / SpO2 counts + maxTs) into the day key, or let
force=true bypass the day cache as users already expect it to.