Fix device_info rewrite for devices that omit product (COROS DURA) — fixes #80

[nothingshocking]

Fixes #80.

Problem

The COROS DURA writes a device_info record containing only manufacturer (294)
and a text product_name. It has no numeric product, device_index,
serial_number, or source_type. The existing rewrite only overwrites product
when it is already present, so the output has manufacturer=GARMIN with no product,
and Garmin Connect shows no device for the activity.

Simply removing the is not None guard isn't enough: a parsed message's fields are
bound to the source definition, so assigning a field that wasn't defined raises
Field is not growable.

Fix

When a device_info record being modified has no product/garmin_product:

• Rebuild it as a fresh DeviceInfoMessage (same approach as the FileIdMessage
  rewrite), copying all populated fields across.
• Set product to the profile's target device.
• Fill device_index=0 (creator), source_type=local, and serial_number (from the
  profile, falling back to the existing default) if absent, so Garmin Connect
  recognises the record as the recording device.

Records that already carry a product (e.g. Hammerhead Karoo) go through the existing
code path unchanged.

Testing

• Two regression tests added in tests/test_fit_editor.py (synthetic DURA-style files).
• Full suite: all pass except test_show_dirs_flag_with_garmin_dirs, which also fails
  on unmodified main in my environment.
• Real device: re-uploaded a COROS DURA activity (Aug 4, 2026). Before: no device in
  Garmin Connect. After: shows Edge 830. Karoo activities upload unchanged (409
  duplicate as expected).