Performance (hot path in receive()):
- Switch port reads from read(1) — which allocates a new bytes object on
  every call — to `readinto(self._read_buf)`, reusing the bytearray already
  allocated in `__init__` but previously unused
- Cache `self._in_port` and `self._read_buf` as locals to avoid repeated dict
  lookups on self inside the tight loop
- Replace two `_LEN_*_MESSAGES` set membership checks with a single
  `_MSG_DATA_LEN.get()` dict lookup to determine data byte count
- Inline data-byte reads and corruption checks; remove the tuple allocation
  that the old validation loop created on every call
- Use a module-level `_msg_buf` scratch bytearray in `__bytes__()` instead of
   `bytes([...])`, avoiding a list allocation per serialization

 Correctness:
 - Fix SysEx desync: SYSEX was in `_LEN_0_MESSAGES` so `receive()` returned
   immediately after the 0xF0 status byte, leaving the variable-length
   payload in the port buffer and causing all subsequent receive() calls to
   fail with error_count increments. Now the parser consumes bytes until
   0xF7 before returning Message(SYSEX).
- Fix channel-0 silent drop: `send(msg, channel=0)` was a no-op because
  `if channel:` is falsy for 0. Changed to `if channel is not None:`.
- Fix pitch bend full-down corruption: `Message.__init__` triggered the
  user-API shortcut (treating data0 as a full pitchbend value) whenever
  data1==0, which corrupted wire bytes [0xE0, 0x00, 0x00] (-8192) into
  [0xE0, 0x00, 0x40] (center 0). The condition now also requires data0
  to be outside the valid raw-MIDI-byte range (0–127).

examples/tmidi_test_device.py (CircuitPython, copy to device as code.py):
- Echo loop — receives any MIDI message and sends it back, except SysEx which is consumed silently
- LED blinks on each message; prints each message to the REPL with running error count
- USE_USB_MIDI = True (default) for USB MIDI; set to False + add usb_cdc.enable() in boot.py for the serial path

examples/tmidi_test_host.py (CPython):
- --usb-midi mode uses mido + python-rtmidi; auto-detects "circuit/pico" in port names, or --port NAME to specify
- --serial PORT mode uses pyserial; sends raw MIDI bytes and parses the echo stream
- 9 tests: NOTE_ON/OFF, CC, Program Change, pitch bend (center / full-up / full-down), Clock, and the SysEx desync test
- Pass/fail per test with elapsed ms; exits with code 0 on all-pass, 1 on any failure

… would need for MIDI messages