Infer charging from a rising level on Bluetooth devices

[PyFlat-JR]

What does this PR do?

Shows Bluetooth devices as charging when their level keeps rising.

• Bluetooth only reports a level, never whether a device charges, so BluetoothBatterySource always reported Discharging. It now marks that state as a guess (BatteryReading.StatusIsAssumed); an Unknown state counts the same.
• ChargingInference runs in the poll loop before the registry, so the widgets, the charging and status variables, the trend and the charging started/stopped events all see the same state.
• Starts once the level is at least 2 points above its lowest in the last 15 minutes. A device that reports in 10 % steps passes on its first step; 1 % jitter doesn't.
• Ends on any drop below the peak, or after 30 minutes without a rise, so a device left at 100 % or taken off the charger doesn't stay charging. The history then restarts so the old low can't re-trigger it.
• A state the source reported itself is never replaced.

Checklist

• I have read AGENTS.md and CONTRIBUTING.md.
• dotnet build and dotnet test pass locally (0 warnings, 265 tests).
• I updated README.md / AGENTS.md if this changes what they describe.
• If this touches a device backend: I tested it against the real hardware (see below).

Device/hardware tested against (if applicable)

The rules are covered by unit tests with a fake clock.