refactor: one Platform ABC — collapse 5 abstractions into single OS foundation

Delete PathResolver, PlatformSetup, PlatformAdapter, OSConfig, and the
delegate-only Platform class. Replace with one Platform ABC: shared logic
on the base (paths, screen capture, sensor caching), each OS overrides
what differs.

- 4 OS files inherit Platform directly, no phantom OSPlatform
- ScsiProtocol follows UsbProtocol pattern: lazy transport creation
- Dead code removed: create_hid/bulk_transport, list_usb_devices,
  discover_hardware, get_metrics (never called by consumers)
- MockPlatform is a proper Platform subclass with noop USB
- All consumers reference Platform, not deleted types
- 5725 tests pass, 0 new failures

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>