libpebble3: recognise the PebbleOS emulator platforms (#285)

The PebbleOS QEMU boards report their own hardware platforms (0xf6
flint, 0xf5 emery, 0xf2 gabbro). They were unknown, so the watch type
fell back to unknownWatchTypePlatform (emery by default): an emulated
flint connected over Bluetooth got emery app binaries and drew them
off-screen, and firmware update checks failed with "Unknown platform".

Map each emulator to its watch type.

Related: coredevices/pebbleos#2142

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>