[ps2]

Fixes #111. Supersedes #112, which keeps the alert marker but still re-adopts the sensor.

What happened

Seen on a phone eavesdropping on a G7 (DXCMgc, 14 days 17 hours into a 15-day session). The sensor reported state 25 (sensorFailed), and from then on every five minutes:

1. Loop raised Sensor Failed, then forgot the sensor and scanned for a replacement. The forget cleared sensorFailedAlertIssuedFor and retracted the alert the moment it was issued.
2. The scan connected to the only Dexcom sensor in range to identify it: the same failed one. Discovery adopts any sensor, so it became a "new sensor", with a new paired date, rescheduled expiry alerts and a sensor start event.
3. Its next message was the same failed reading, so the alarm, the sensor end event and the forget repeated. The previous-sensor record was overwritten each time.

The user acknowledged three critical alarms in ten minutes, and they continue until the sensor is out of range.

Change

• G7SensorRecord.EndReason gains failed and sessionEnded, and the Previous Sensor page shows them. The failure and session-ended paths forget the sensor with that reason; scanForNewSensor() (the user's Scan for New Sensor) still uses replaced.
• Discovery declines a sensor whose previous record ended as failed or session ended, matched by name and activation time (within a minute). A sensor the user forgot can still be adopted again.
• Sensor Failed stays up after a failure forget, and is retracted when a different sensor is adopted. sensorFailedAlertIssuedFor survives the forget, as in Suppress repeated G7 sensor failure alerts during replacement scans #112.

Tests

New in G7LifecycleAlertManagerTests:

• a failed sensor rediscovered three times: one alert, not retracted, no further sensor start/end events, and the record keeps failed and its failure message
• a different sensor retracts the failure alert and is adopted
• a sensor whose session ended is not adopted again
• a sensor forgotten with Scan for New Sensor can be adopted again

All 207 G7SensorKit tests pass on the iOS Simulator.