feat(teensyrom): cross-platform USB-serial auto-detection

[kfox]

What

autodetect_serial_port() was macOS-only: it shelled out to system_profiler
for the board's USB serial number, then globbed /dev/cu.usbmodem<serial>* for
the node. Off macOS it hard-returned None, so tr:// (auto-detect) required an
explicit [teensyrom].serial_port on Linux and Windows.

This replaces that with a single pyserial list_ports.comports() path that
works on macOS, Linux, and Windows. The board is identified by its USB
(VID 0x16C0, PID 0x0489) — the only stable, driver-independent key
across platforms — with a case-insensitive "teensyrom" product/description
substring accepted as a fallback where the OS surfaces the USB product string.
Each comports() entry already carries the resolved device path, so there's no
per-platform node resolution: it returns COMn / /dev/ttyACM* /
/dev/cu.usbmodem* directly. pyserial is already the tr extra dependency.

The generic Windows usbser.sys driver exposes no product string (product is
None, description is the bare USB Serial Device (COMn)), which is why the
(VID, PID) key leads and the product-name match is only a fallback.

Changes

• teensyrom_dma.py: rewrite autodetect_serial_port() on list_ports; add a
  pure, duck-typed _is_teensyrom_port() matcher + _TEENSY_USB_VID /
  _TEENSYROM_USB_PID constants; isolate the pyserial call behind
  _list_comports() (returns [] when pyserial is missing or enumeration
  fails); drop the dead system_profiler/glob machinery and its
  glob/json/subprocess/sys imports.
• backend.py: drop the "macOS only" wording from the auto-detect log comment
  and the missing-serial_port error message.
• scripts/diags/tr_serial_probe.py: new diagnostic — runs the same
  enumeration and prints the environment, each port's fields, and the match
  decision, so a failed auto-detect explains itself instead of a bare None.
• Tests: replace the profiler/glob unit tests with matcher cases (VID/PID with
  no product string, product-string fallback, rejection) and _list_comports-
  mocked autodetect tests incl. the multi-board warning. The tests patch
  _list_comports, so they need no pyserial (the tr extra) installed.
• CLAUDE.md: note auto-detection is now cross-platform via USB VID/PID.

Verification

• ruff check, ruff format, mypy --strict, pyright, full unittest suite
  (1902 tests) all pass — including with pyserial absent (the CI extra set).
• Auto-detect confirmed resolving the real board's device path on both macOS and
  Windows (via the new diagnostic); the resolved path feeds the unchanged
  SerialTransport.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.65%. Comparing base (63da329) to head (387d19d).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
c64cast/teensyrom_dma.py	80.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
+ Coverage   79.56%   79.65%   +0.08%     
==========================================
  Files          76       76              
  Lines       15426    15403      -23     
  Branches     2321     2315       -6     
==========================================
- Hits        12274    12269       -5     
+ Misses       2622     2607      -15     
+ Partials      530      527       -3     

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