feat(video): add bounded packet history, delayed FIFO and owned snapshots

packages/video/docs/packet-ring.mdadded+36-0

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

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	1	+# Keyframe-aligned packet history
	2	+
	3	+`core/packet_ring.zig` (exported as `video.packet_ring`) adapts the existing video
	4	+ring-buffer structure to encoded pre-roll: storage consists of one fixed byte
	5	+arena and a fixed entry array. Push copies borrowed packets without allocating
	6	+or blocking. The caller serializes all access and chooses timestamp units.
	7	+
	8	+Byte capacity, frame capacity and duration are hard upper bounds. Eviction
	9	+removes dependent packets until the oldest retained packet is a keyframe.
	10	+If the limits cannot hold a GOP, history can be empty until the next key.
	11	+Oversized packets and non-increasing timestamps are rejected without modifying
	12	+retained history. Parameter sets/codec configuration belong outside the ring.
	13	+`entry` returns metadata; `copy` explicitly copies a retained payload to caller
	14	+storage, including when it wraps in the byte arena. Views expire on mutation.
	15	+
	16	+Three standalone regression tests cover arena wrap, byte/frame/time eviction,
	17	+keyframe gating, rejected-input atomicity, zero capacities and allocator failure
	18	+cleanup. Tested on Zig 0.15.2 and the installed 0.17 development toolchain. This
	19	+focused validation does not establish correctness of the wider video graph.
	20	+
	21	+zig-camera uses this module with Home JPEG/Matroska and x264/x265/fragmented MP4.
	22	+It retains compressed access units, without retaining camera DMA leases or raw
	23	+RGB frames. Explicit snapshots need scratch space for one largest retained
	24	+packet; remuxing starts with fresh codec headers and resets file timestamps.
	25	+
	26	+For time-shifted streaming, `pushQueued` uses the same owned arena as a bounded
	27	+FIFO: it never evicts or filters dependent frames, and rejects full/oversized or
	28	+non-monotonic input without mutation. Use `ready(now, delay)` then `copy(0, scratch)`
	29	+and `pop()` to forward matured packets; `ready(null, delay)` flushes EOF. Queue
	30	+capacity must cover the delay. The caller controls keyframe starts and output
	31	+errors; a full FIFO must not silently discard encoded dependencies.
	32	+
	33	+`cloneFrom(allocator, start)` owns a minimally sized copy of a selected suffix.
	34	+Serialize it with source mutation; the resulting data can move to a worker and
	35	+outlive the source. It copies wrapped bytes directly into a contiguous arena and
	36	+cleans up partial allocation failures. The module creates no threads or file I/O.