test(flexiband): self-terminate demux at first malformed frame

[zsoerenm]

What

The III-7a integration test now stops the demux at the first malformed USB frame instead of reading all the way to EOF.

Why

The Flexiband III-7a capture is only well-formed for its first ~13.68 s; the trailing ~2 s has broken 0x55 0xAA preambles and demuxes to noise. This is the same capture-tail artifact diagnosed in Tracking.jl #157 / #158.

iii7a_read_bands! did not validate the frame preamble, so it fed those garbage tail frames into track!/decode.

Change

• Check the 0x55 0xAA preamble on each frame; stop at the first bad one and return only the valid prefix. When a window comes back short, the next read returns empty and the existing isempty(l1) && break ends the tracking loop cleanly.
• Document that the demux is already windowed — one 0.2 s chunk read straight from the decompressing zip stream, never materializing the full ~1.93 GB .usb, so memory stays bounded (lighter than #158's retain-then-@view approach; no OOM risk here).

Notes

The integration test is opt-in (GNSSDECODER_RUN_INTEGRATION_TEST=true) and downloads a 1.6 GB capture, so it was not run end-to-end here. The preamble logic mirrors the verified read_payload in Tracking.jl #158. Change is JuliaFormatter 2.8.5 clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.67%. Comparing base (8aa692a) to head (1d77958).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #59   +/-   ##
=======================================
  Coverage   97.67%   97.67%           
=======================================
  Files          10       10           
  Lines        1336     1336           
=======================================
  Hits         1305     1305           
  Misses         31       31           

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