Add StopWorkout command with safe treadmill stop sequencing, serialize HID I/O, and improve logging

[cagnulein]

Motivation

• Introduce an explicit StopWorkout command to distinguish a connected treadmill stop sequence from a simple pause and to avoid racing with resume/target writes.
• Prevent unsafe interleaving of multi-step safety commands and poll I/O that could leave belt machines running or MCU USB state inconsistent.
• Improve observability for safety-critical writes and device handshake/poll mismatches.

Description

• Added DeviceCommand.StopWorkout to the sealed DeviceCommand hierarchy.
• Implemented serialized command submission using commandMutex and serialized HID request/response transactions with ioMutex, and routed writeFeature through writeFeatureSerialized.
• Implemented a belt-specific safe stop flow in V1Session (stopWorkoutSafely) that discards pending writes, forces KPH=0 + WORKOUT_MODE=PAUSE, confirms belt halt via haltBeltConfirmed(reason), writes WORKOUT_MODE=IDLE and optionally confirms it, and kept USB session connected; gracefulEndForDisconnect updated to call haltBeltConfirmed("disconnect").
• Made pollOnce run under ioMutex and added logSafetyWrites to log explicit writes for WORKOUT_MODE, KPH, IDLE_MODE_LOCKOUT, and REQUIRE_START_REQUESTED; added additional console-init / transition-to-active handling to avoid starting treadmills implicitly.
• Treated StopWorkout as equivalent to pause on V2 devices in V2Session, and updated FitProDeviceService.stopWorkout() to send DeviceCommand.StopWorkout.
• Moved device-originated UI slots (StartFromDevice, PauseFromDevice, StopFromDevice) to public slots in homeform.h to allow device implementations to invoke them without echoing commands to the physical console.
• Misc: added extra logging for handshake supported bitfields and data-response truncation diagnostics.

Testing

• Built the Android app and ran the project's unit test suite; build and unit tests completed successfully.
• Ran integration smoke checks for V1/V2 session command flows (start/pause/stop/resume) in a development environment; stop/resume serialization and logging behaved as expected.
• No automated tests failed.

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Codex Task