feat(video): add checked capture plane packing

packages/video/docs/capture-planes.mdadded+23-0

Changes to packages/video/docs/capture-planes.md

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	1	+# Packing captured image planes
	2	+
	3	+`core/plane_pack.zig` provides allocation-free checked copies for capture buffers
	4	+whose row padding is not part of the image. It is also exported as
	5	+`video.plane_pack`.
	6	+
	7	+- `rows(source, stride, active_row_bytes, height, destination)` copies active bytes.
	8	+- `packI420(planes, strides, width, height, destination)` packs separate Y, U, V
	9	+ planes into tight I420; each source plane can have its own stride.
	10	+
	11	+Both validate sizes with checked arithmetic before writing. Final row padding
	12	+need not be present. I420 requires positive even dimensions. Truncated chroma,
	13	+undersized output and overflow are errors; the destination remains unchanged.
	14	+Sources remain borrowed, the caller owns the destination, and source and output
	15	+must not overlap. These helpers do not convert color, resize, or synchronize DMA.
	16	+
	17	+zig-camera uses these helpers after beginning CPU access to a leased DMA buffer.
	18	+The camera ISP produces its low-resolution stream; packing only happens for an
	19	+explicit owned copy. A mapped view keeps the original padding and ownership.
	20	+
	21	+Focused validation: `zig test packages/video/src/core/plane_pack.zig` on Zig
	22	+0.15.2 and Home's 0.17 development toolchain. This does not establish correctness
	23	+of the entire video package's unrelated codecs or conversion paths.