Parse G7 glucose messages with a nonzero status

[ps2]

Backport of the parsing fix from #85 to main, cherry-picked from d3f2a7b.

A failed sensor answers the glucose request with status 0x80, its last reading and algorithm state 27 (transmitterFailed). The parser rejected any nonzero status. So the manager logged "Unable to handle glucose control response" every five minutes, never learned the state, and showed the sensor as connecting instead of raising the sensor-failed alert.

The message now keeps the status and decodes the timestamp, sequence, age and algorithm state for any status. Glucose, prediction and trend are nil when the status is nonzero, so a reading the sensor didn't mark current is never stored. On main, G7CGMManager.sensor(_:didRead:) checks algorithmState.sensorFailed before it looks at glucose, so the alert fires as on next-dev.

Not included

#85's other three commits (asking for the transmitter version until the serial number arrives, and sending the serial with the sensor start) depend on next-dev-only code: the transmitter version query, G7SensorRecord, and the previous-sensor page. They aren't part of this backport.

Testing

Adds testFailedSensorStatusStillReportsAlgorithmState to G7GlucoseMessageTests. The same change and test pass on next-dev. I couldn't build the legacy workspace here: Xcode 27 rejects its watchOS 7.1/6.1 and TrueTime iOS 12 deployment targets before anything compiles.

[ps2]

This is not an important thing to fix for non-direct integration.