Don't re-evaluate glucose alerts from a backfill older than the last reading

[ps2]

CGMs deliver the live reading and then backfill the gap behind it, each as its own .newData batch, and DeviceDataManager hands every batch to GlucoseAlertManager.evaluate(samples:). The backfill carries a sample no newer than the one just evaluated, and evaluate re-decided the alert from it.

Seen with a G7 on next-dev, from the AlertStore and DeviceLog:

22:58:59  didRead        glucose 86  timestamp 89595   → evaluate → urgent low issued
22:58:59  didReadBackfill glucose 86  timestamp 89591   → evaluate → urgent low retracted 22:59:00

Retraction goes through unscheduleAlertWithSchedulers, which also calls criticalAlertAudioPlayer.stop() — so on an un-entitled build the in-process alarm audio was cut off one second after it started, and the notification was pulled with it. The user saw a banner slide in and out and heard nothing.

Fix

Skip any batch whose newest sample is not newer than the last evaluated reading. Backfill still reaches the glucose store; it just cannot override an alert decision that was made on a newer reading.

Testing

Builds clean. The sequence above is reproducible with the "next reading as urgent low" test on a G7, which is how it was found; with the guard the backfill batch is logged and ignored.

[ps2]

Folding this into #2520 — same fix set, one PR.