keychron: let the M6 set its DPI stage count

[ydw1904]

What

The M6 now reports `countEditable: true` and implements `setDpiStageCount`, so the shared stage editor shows the stage count picker (1 to 5) instead of hiding it.

Why

The settings packet already carries the stage count at byte 14 and `parseStatus` already reads it back, so the wire format was known. Only the setter was missing, which kept the picker hidden.

Details

• All five hardware slots keep their DPI; the count only decides how many the DPI button cycles through.
• The active stage rides on the same packet, so it is clamped below the new count before writing. Shrinking past it would otherwise write a stage the mouse cannot hold.
• The write is confirmed by reading the settings back; a mismatch throws.

Testing

• `npm run build` passes
• `tsx --test src/drivers/keychron/*.test.ts`: 41/41 pass, including new tests for the setter, the active stage clamp, and out of range counts

Not yet exercised on hardware: until now the count field was only ever written back with the value just read. The app needs no change, `applyDpiStageCount` already gates on `countEditable` and `setDpiStageCount`.