Fixes three RTMP chunk-stream decoding bugs in the proxy and hardens AMF0 command-packet unmarshalling against malformed input, backed by a new protocol unit-test suite.
All changes are confined to the `internal/rtmp` package. No public API, log format, or emitted wire format changes — these are decode-correctness and robustness fixes only.
**3-byte chunk basic header decode (`readBasicHeader`) **
The 3-byte basic-header form (cid 64–65599) was selected by testing `cid == 1` *after* `cid` had already been overwritten with `64 + t`, so it was never detected. Capture the original marker before overwriting and test that instead.
**Extended-timestamp handling (`chunkStream`, `readMessageHeader`)**
- Use the extended timestamp as a delta for fmt=1/2 chunks (and a fmt=3 first chunk continuing them), required when the delta is ≥ `0xffffff`. Timestamp computation is unified into a single post-step: extended timestamp when present, otherwise the 3-byte header delta; fmt=0 absolute, fmt=1/2 accumulated.
- Detect Type-3 chunks that omit the extended timestamp. FMLE/FMS/Flash follow the RTMP 2012 spec and always send it on Type-3 chunks; librtmp/ffmpeg may not. Switched from an unconditional 4-byte read to `Peek` + conditional `Discard`: if the peeked value differs from the stored one on a non-first chunk, those 4 bytes are payload and are left in the reader.
- Split the single `extendedTimestamp` bool into `hasExtendedTimestamp` (bool) and `extendedTimestamp` (the last raw value, used for the detection above).
**Packet unmarshal hardening**
- Add an `advanceBytes(p, n)` helper that bounds-checks each `p = p[field.Size():]` advance, turning a slice-out-of-range panic into a clean error on truncated/untrusted input. Applied in `CallPacket`, `CreateStreamResPacket`, `PublishPacket`, and `PlayPacket`.
- Reset the optional `CommandObject` / `Args` to nil before probing for their presence, so a stale constructor default (e.g. Null) isn't counted by `Size()` and can't overflow a later advance.
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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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