Make V4L2 dequeue the primary ingest path; demote EOI scanner

ingest module: dequeue V4L2 buffers, emit one encapsulated frame per buffer.
Driver guarantees per-buffer framing for V4L2_PIX_FMT_MJPEG; no scanning needed.

mjpeg_scan: future optional module for non-compliant hardware only.
Explicitly a workaround, not part of the primary pipeline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

planning.md

@@ -50,11 +50,12 @@ Modules are listed in intended build order. Each depends only on modules above i
 | 6 | `protocol` | not started | Typed `write_*`/`read_*` functions for all message types; builds on serial + transport |
 | 7 | `frame_alloc` | not started | Per-frame allocation with bookkeeping (byte budget, ref counting) |
 | 8 | `relay` | not started | Input dispatch to output queues (low-latency and completeness modes) |
-| 9 | `ingest` | not started | MJPEG frame parser (two-pass EOI state machine, opaque stream → discrete frames) |
+| 9 | `ingest` | not started | V4L2 capture loop — dequeue buffers, emit one encapsulated frame per buffer |
 | 10 | `archive` | not started | Write frames to disk, control messages to binary log |
 | 11 | `codec` | not started | Per-frame encode/decode — MJPEG (libjpeg-turbo), QOI, ZSTD-raw, VA-API H.264 intra; used by screen grab source and archive |
 | 12 | `xorg` | not started | X11 screen geometry queries (XRandR), screen grab source (calls codec), frame viewer sink — see architecture.md |
 | 13 | `web node` | not started | Node.js/Express peer — speaks binary protocol on socket side, HTTP/WebSocket to browser; `protocol.mjs` mirrors C protocol module |
+| — | `mjpeg_scan` | future | EOI marker scanner for misbehaving hardware that does not deliver clean per-buffer frames; not part of the primary pipeline |
 
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