On this device the NG link is intermittent — a read/download that returns clean samples
one minute comes back as desynced BigBuf memory the next, and lf.read()'s reported count
is sometimes garbage (so sizing the download off it over-reads into memory). Rather than
trust either, capture now:
- downloads a fixed safe span (_DOWNLOAD_BYTES) instead of the flaky read-count,
- validates the result is an envelope, not memory (rejects the 0xDEADBEEF stack canary,
an embedded PM3 response magic, or a mostly-zero unwritten buffer),
- retries the read+download a few times, giving up cleanly to b'' if the device only
returns memory (needs a power-cycle / lower debug level).
read_emitted/read_config go through this, so identify recovers from transient desyncs
instead of demodulating garbage. Tests: validator rejects memory patterns; capture retries
past a garbage download to the good one, and gives up when all attempts are garbage.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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