ffmpeg-v4l2-request-fourier: per-MB inspection callback for H.264 (0016)

Adds 0016-h264-mb-inspect-callback.patch to the FFmpeg fork.  Adds an
opt-in callback fired by ff_h264_hl_decode_mb after the existing
pixel work, for tools that need per-MB visibility into H.264 decode.

API:
  typedef void (*ff_h264_mb_inspect_cb)(void *opaque,
                                         const struct H264Context *h,
                                         int mb_x, int mb_y);
  void ff_h264_set_mb_inspect_cb(AVCodecContext *avctx,
                                  ff_h264_mb_inspect_cb cb, void *opaque);

Two new fields appended to H264Context (internal struct, declared in
h264dec.h not h264.h, no ABI surface to non-libavcodec callers).
Callback fires post-pixel-work for every MB in coded order; receives
const H264Context* so it can inspect any state (slice ctx via
h->slice_ctx, reconstructed pixels via h->cur_pic.f->data[plane],
etc.).

Default (cb==NULL): zero behaviour change, one load + one branch per
MB in the decoder hot path.

Shape distinction: per-MB observation, NOT per-kernel function-pointer
hijack (the 0003-0014 substitution-arc pattern that PR #105 reverted
+ daedalus-fourier PR #37's measurement-correction architecturally
retired).  Per-block synchronous Vulkan dispatch from libavcodec is
non-competitive; per-MB CPU-side observation feeding a per-frame
daedalus-decoder batch submit is the right shape (frame-major UMA
dispatch verdict, memory: dejavu).

Used by:
  - daedalus-decoder/tools/daedalus_decode_h264 (PR-A1b, follow-up)
  - future daedalus-v4l2 daemon refactor

Wired into arch PKGBUILD source[] + prepare() and debian build-deb.sh
patch sequence.  pkgrel bumped 12 → 13.

Refs reauktion/daedalus-decoder!12.

debian/ffmpeg-v4l2-request-fourier/0016-h264-mb-inspect-callback.patch

@@ -0,0 +1,115 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Markus Fritsche <mfritsche@reauktion.de>
+Date: Tue, 26 May 2026 06:00:00 +0200
+Subject: [PATCH] avcodec/h264: per-MB inspection callback (daedalus-decoder
+ hook)
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+Adds an opt-in callback fired in ff_h264_hl_decode_mb after the
+existing pixel work, used by tools that need per-MB visibility into
+the H.264 decode.  Initially driven by daedalus-decoder's CLI test
+harness (tools/daedalus_decode_h264) which shadows libavcodec's
+decode with a frame-major daedalus-decoder run for byte-exact diff
+on real H.264 streams; later target is a daedalus-v4l2 daemon
+refactor that drives daedalus_decoder_append_mb directly from the
+callback instead of letting libavcodec do per-MB pixel work.
+
+Shape: ONE inspection point per MB.  Distinct from the per-kernel
+function-pointer-hijack pattern that used to live in 0003-0014
+patches (now reverted via 0015 for ctx, and architecturally retired
+per daedalus-fourier PR #37's measurement-correction).  Per-block
+synchronous Vulkan dispatch from libavcodec was structurally non-
+competitive; per-MB CPU-side observation feeding a per-frame batch
+submit is the right shape.
+
+Two new fields in H264Context (appended at end of struct; no ABI
+surface visible to non-libavcodec callers since H264Context is
+internal — declared in h264dec.h, not h264.h).  One new exported
+function ff_h264_set_mb_inspect_cb to set them.
+
+Zero behaviour change when cb == NULL (the default): one load +
+one branch per MB in the decoder hot path, both branch-predicted
+to fall through.
+
+Used by:
+  - daedalus-decoder/tools/daedalus_decode_h264 (PR-A1b)
+  - future daedalus-v4l2 daemon refactor
+
+Refs reauktion/daedalus-decoder!12 (Stage 2 PR-b complete).
+---
+ libavcodec/h264_mb.c | 20 ++++++++++++++++++++
+ libavcodec/h264dec.h | 26 ++++++++++++++++++++++++++
+ 2 files changed, 46 insertions(+)
+
+--- a/libavcodec/h264dec.h
++++ b/libavcodec/h264dec.h
+@@ -334,6 +334,16 @@
+     int pic_order_cnt_bit_size;
+ } H264SliceContext;
+ 
++/* Per-MB inspection callback type — see ff_h264_set_mb_inspect_cb()
++ * below.  Fired by ff_h264_hl_decode_mb after the existing pixel work
++ * for every macroblock in coded order.  Receives a const H264Context*
++ * so the callback can inspect any slice/picture state (h->slice_ctx
++ * for current slice, h->cur_pic.f->data[plane] for reconstructed
++ * samples, etc.). */
++typedef void (*ff_h264_mb_inspect_cb)(void *opaque,
++                                       const struct H264Context *h,
++                                       int mb_x, int mb_y);
++
+ /**
+  * H264Context
+  */
+@@ -579,6 +589,10 @@
+     int non_gray;                       ///< Did we encounter a intra frame after a gray gap frame
+     int noref_gray;
+     int skip_gray;
++
++    /* Per-MB inspection hook — set via ff_h264_set_mb_inspect_cb. */
++    ff_h264_mb_inspect_cb mb_inspect_cb;
++    void                 *mb_inspect_opaque;
+ } H264Context;
+ 
+ extern const uint16_t ff_h264_mb_sizes[4];
+@@ -607,6 +621,16 @@
+                                    const H2645NAL *nal, void *logctx);
+ 
+ void ff_h264_hl_decode_mb(const H264Context *h, H264SliceContext *sl);
++
++/**
++ * Install an opt-in per-MB inspection callback that fires from
++ * ff_h264_hl_decode_mb after each macroblock's pixel work.  Default
++ * is NULL (no callback installed); the check is a single branch on
++ * the decoder hot path.  See ff_h264_mb_inspect_cb for signature.
++ */
++void ff_h264_set_mb_inspect_cb(AVCodecContext *avctx,
++                                ff_h264_mb_inspect_cb cb, void *opaque);
++
+ void ff_h264_decode_init_vlc(void);
+ 
+ /**
+--- a/libavcodec/h264_mb.c
++++ b/libavcodec/h264_mb.c
+@@ -815,4 +815,20 @@
+         hl_decode_mb_simple_16(h, sl);
+     } else
+         hl_decode_mb_simple_8(h, sl);
++
++    /* Per-MB inspection callback (opt-in via ff_h264_set_mb_inspect_cb).
++     * Fired AFTER pixel work — reconstructed samples are in
++     * h->cur_pic.f->data[plane] at the MB's raster position by the
++     * time this runs.  Callback may inspect slice context via
++     * h->slice_ctx + sl->mb_xy, coeffs via sl->mb, etc. */
++    if (h->mb_inspect_cb)
++        h->mb_inspect_cb(h->mb_inspect_opaque, h, sl->mb_x, sl->mb_y);
++}
++
++void ff_h264_set_mb_inspect_cb(AVCodecContext *avctx,
++                                ff_h264_mb_inspect_cb cb, void *opaque)
++{
++    H264Context *h = avctx->priv_data;
++    h->mb_inspect_cb     = cb;
++    h->mb_inspect_opaque = opaque;
+ }