readBasicHeader overwrote cid with the 2-byte form (64 + byte2) before testing
whether the 3-byte form was in use, so the `cid == 1` check could never be true
and the 3-byte branch was dead code. Chunk basic headers with marker == 1 (chunk
stream IDs 320-65599) consumed only one of the two trailing bytes, leaving the
high-order byte in the stream and desyncing the chunk parser.
Keep the original marker before cid is overwritten and branch on it, matching the
C++ reference (srs_protocol_rtmp_stack.cpp, read_basic_header). The arithmetic
inside the branch was already correct.
Also correct the unit test, which had encoded the buggy result (expected cid=65
instead of 577, leaving a byte unread); it now guards the 3-byte path.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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