Battery alert: stop re-firing the low-battery notification while charging (#80)

[ryanbr]

Fixes #80 — the "charge your strap" low-battery notification fires repeatedly while the strap is charging.

Root cause (both platforms — identical policy)

BatteryAlertPolicy already knew about charging, but the two conditions interacted badly:

• re-armed the low flag on charging == true, yet
• fired whenever charging != true (which includes the null/unknown state).

The strap reports its charge bit only on BATTERY_LEVEL events (~every 8 min), so charging flickers true → null between events:

1. true read → re-arms low = false (no fire).
2. null gap → !low is true, pct ≤ 15, charging != true is true → fires again.

Every flicker re-armed then re-fired → repeated notifications while plugged in.

Fix

Re-arm the low flag only on genuine recovery (pct >= lowRearmAbove), not on the charge bit:

- if (charging == true || pct >= LOW_REARM_ABOVE) low = false
+ if (pct >= LOW_REARM_ABOVE) low = false

fireLow's charging != true guard is untouched, so:

• an explicit charging == true read still suppresses;
• a strap that never reports a charge bit (charging == null, e.g. generic/FTMS) still gets low alerts (no regression);
• charging still re-arms for the next cycle — via the recovered percentage once it climbs past 25%, not the flickering bit. A partial top-up that stays < 25% simply won't nag.

Applied identically to iOS (BatteryNotifier) and Android (BatteryAlertPolicy).

Tests

Updated the "charging suppresses low" case (charging must not re-arm now) and added a true→null flicker regression on both platforms. Android unit test green (testFullDebugUnitTest --tests BatteryAlertPolicyTest, exit 0). iOS is the byte-identical mirror (can't run on WSL — StrandTests → GRDB sqlite3.h wall — runs in CI).

Files: Strand/System/BatteryNotifier.swift, StrandTests/BatteryAlertPolicyTests.swift, android/.../BatteryAlertNotifier.kt, android/.../BatteryAlertPolicyTest.kt.