feat(video): add checked elementary packet and PTS outputs

packages/video/docs/packet-output.mdadded+34-0

Changes to packages/video/docs/packet-output.md

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	1	+# Checked elementary packet and PTS output
	2	+
	3	+`src/core/packet_output.zig` is a standalone, allocation-free boundary for
	4	+reusable streaming sinks. The broader sink graph includes decoded-frame and
	5	+accumulating packet collections; this module neither retains borrowed packets
	6	+nor imports that graph.
	7	+
	8	+`annexB(input, output)` converts four-byte big-endian length-prefixed AVC/HEVC
	9	+NAL units to four-byte Annex B start codes. It checks every length before writing
	10	+any destination bytes, rejects empty/truncated/zero-length input, and checks
	11	+output capacity. Input/output must not overlap. Parameter sets can be converted
	12	+separately and prepended at the first key of a newly attached output.
	13	+
	14	+`formatPts(timestamp_us, output)` formats integer microsecond PTS as milliseconds
	15	+with exactly three decimal digits and LF. Zero and all u64 timestamps are exact;
	16	+no floating-point rounding or locale formatting is used. Too-small buffers remain
	17	+unchanged. A 32-byte destination covers the complete u64 range.
	18	+
	19	+`PtsWriter` wraps an explicit synchronous `Sink{context,write}`. Sink callbacks
	20	+must completely accept their borrowed data or return an error. Rejected
	21	+non-increasing timestamps do not touch the sink. A sink failure poisons the
	22	+writer; create a fresh writer for a new clip/epoch. The module does not own files,
	23	+threads or packet storage and does not depend on FFmpeg/Libav.
	24	+
	25	+Focused tests on Zig 0.15.2 and the Home development toolchain:
	26	+
	27	+```sh
	28	+zig test packages/video/src/core/packet_output.zig
	29	+```
	30	+
	31	+Tests cover multi-NAL conversion, malformed/truncated input, exact maximum PTS,
	32	+small-buffer state preservation, monotonicity and sink-failure poisoning.
	33	+zig-camera uses the helpers at its native callback output boundary, with
	34	+independent video decoding and media/PTS timeline checks in its integration suite.