Support Brother PT-P710BT over USB

[pfa230]

Add the Brother PT-P710BT USB print path while keeping PT-2730 as the default. Explicit PTOUCH_MODEL=PT-P710BT selects USB identity 04f9:20af, model-correct queue discovery, extended page information, and 128-dot TIFF PackBits output through the existing driver and finishing lifecycle.

Retain canonical 1:1 geometry, strict width rejection, all four finishing presets, healthy continuation, offline recovery, and conservative uncertain-output recovery. The OpenSpec change is synced and archived with all 21 tasks complete. The KISS review retains two explicit model choices without a new framework, planner, buffer, or dependency.

Validation:

• Pinned full configure/build/CTest: 12/12 passed; core-only: 6/6 passed.
• Deployment-image IPP/socket smokes passed for both PT-2730 and PT-P710BT; CI runs both before publication.
• PT-2730 baseline/candidate production-callback output is byte-identical: 4,180,474 bytes, SHA-256 cc9986ca8e2beaea34ad7102c42867e887ebc6d3533fd1b7d6d797ab2046c4b4.
• Integrated P710BT hardware acceptance passed with 15 complete labels on 9/12/18 mm, all finishing presets, idle continuation, mixed-height streaming, wrong-media rejection, queued USB recovery, and explicit uncertainty recovery. Physical 6/24 mm coverage remains untested.
• Strict OpenSpec validation and diff checks passed. Hardware observations and evidence locations are recorded in tests/P710BT-ACCEPTANCE.md and the archived validation summary.