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/build-deb.sh

@@ -33,7 +33,7 @@ FFMPEG_VERSION=8.1
 # epoch 2 matches Debian's stock ffmpeg (currently 7:7.1.x in trixie);
 # +rfourier suffix to avoid colliding with upstream/Debian rebuilds.
 PKGVER=2:${FFMPEG_VERSION}+rfourier+gb57fbbe
-PKGREL=12  # pkgrel=12 — REVERT pkgrel=11 ctx flip; daedalus-fourier PR #36 4.30x headline was measurement artifact (PR #37 corrects: QPU 22x SLOWER than CPU)
+PKGREL=13  # pkgrel=13 — per-MB inspection callback (0016) for daedalus-decoder CLI test harness; observation-only, no behaviour change to existing decode path
            # (cycle 9 of the daedalus-v4l2#11 step 2 substitution arc; closes
            # the libavcodec.so substitution sequence 6 IDCT4 / 7 IDCT8 /
            # 8 luma-v deblock / 9 qpel mc20).  Pulls daedalus-fourier PR #2
@@ -82,6 +82,7 @@ patch -Np1 -i "$HERE/0012-h264-qpel-rest-daedalus-fourier.patch"
 patch -Np1 -i "$HERE/0013-h264-deblock-chroma-intra-daedalus-fourier.patch"
 patch -Np1 -i "$HERE/0014-h264-ctx-qpu-capable.patch"
 patch -Np1 -i "$HERE/0015-h264-ctx-revert-to-no-qpu.patch"
+patch -Np1 -i "$HERE/0016-h264-mb-inspect-callback.patch"
 
 # --- daedalus-fourier: fetch + build static .a with PIC, install to a
 # per-build prefix; libavcodec.so links it into the shared object so