Feat: Add codec-signaling module (init_data builder + codec-string formatter)

[ghbogarin]

Summary

Adds a new media/codec_signaling module with two composable, pure helpers
that produce the artifacts a catalog needs to signal a media track's codec:

• moq_codec_init_data_build() — builds the decoder configuration record
  (init_data). Elementary streams are assembled into a record (AVC/HEVC
  Annex B → avcC/hvcC, AV1 OBUs → av1C, OpusHead → dOps); already-formed
  records (avcC, hvcC, av1C, dOps) and AudioSpecificConfig are validated and
  copied through unchanged.
• moq_codec_string_format() — formats the codec string
  (e.g. avc1.64001f, mp4a.40.2, hvc1..., av01..., opus) from that
  same record.

Both are allocation-free, thread-safe, and take the byte format explicitly
(no sniffing). Output goes to a caller-provided buffer using the two-call
size protocol (call with buf == NULL to learn the required length; no
partial output on failure).

Supported formats

• AVC — Annex B SPS/PPS → avcC, or pass-through avcC
• HEVC — Annex B VPS/SPS/PPS → hvcC, or pass-through hvcC
• AV1 — OBUs → av1C, or pass-through av1C
• AAC — AudioSpecificConfig (validate + copy)
• Opus — OpusHead (RFC 7845) → dOps, or pass-through dOps

Assembled records are verified byte-for-byte against ffmpeg's muxers, with
documented exceptions (hvcC min_spatial_segmentation_idc emitted as 0;
av1C copies the sequence header verbatim as configOBUs). Unsupported inputs
return MOQ_ERR_UNSUPPORTED rather than producing incorrect output.

Changes

• media/codec_signaling/include/moq/codec_signaling.h — public API
• media/codec_signaling/src/codec_signaling.c — implementation
• media/codec_signaling/tests/test_codec_signaling.c — test suite
• media/codec_signaling/CMakeLists.txt — library + test target
• CMakeLists.txt — new MOQ_BUILD_CODEC_SIGNALING option (ON) wiring the
  subdirectory in

Testing

test_codec_signaling covers both helpers across all supported formats:
happy-path expected-bytes checks, round-trips (build → format), buffer-size
protocol, and error paths (NULL args, malformed/incoherent inputs,
unsupported streams). Registered with CTest as codec_signaling.

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