[ps2]

Lets the Loop watch app read a G7 itself, so readings carry on when the phone isn't there.

With Read on Apple Watch on in the G7 settings, the phone puts the G7's exported configuration in the watch context. The watch builds its own G7 manager from it, in the sensor's watch slot, keeps it across launches, and lets go of the sensor when the phone stops sharing.

• WatchCGMHost hosts the manager. It rebuilds only when the export names a different sensor or code, and it starts at launch so a relaunch for the sensor's link finds it.
• The watch's readings go into its glucose store beside the phone's. The same reading from both carries the same sync identifier, so the store keeps one. The glance and complications show the watch's reading while it's newer than the phone's.
• Glucose backfill from the phone starts no later than the phone's own last reading, so the watch's readings don't hide a gap.
• Bluetooth wakes are handled on the main queue (watchOS can deliver them on CoreBluetooth's queue). A Bluetooth wake is held for 25 s, through the reading. A background refresh 60 s after each reading re-checks that a connect is lodged. It comes after the G7's 35 s wait, so it never connects into the sensor's advertising tail.
• A third page, shown while the watch reads a sensor, shows its name, the G7's note when it needs the code or is searching, the last reading and connection, and a countdown to the next reading.
• The watch app links and embeds G7SensorKit and LoopKit, and declares bluetooth-central with a Bluetooth usage description.

Needs LoopKit/LoopKit#626 and LoopKit/G7SensorKit#80. The registry, the context field and the project entries use the names and IDs of the Sport Mode Loop PR (#2545), which can drop its own copies once this lands.

Tested on an Apple Watch Series 9 with a real G7: the watch paired from the phone's export and read every window in the background with the phone's Bluetooth off.