ffmpeg-v4l2-request-fourier: preserve sl->mb for inspection callback (0017)

Companion to 0016 (PR #106).  Adds a coefficient side buffer in
H264Context, populated at the start of ff_h264_hl_decode_mb with a
single memcpy from sl->mb BEFORE IDCT-add zeros it.  The existing
post-pixel-work callback (still in 0016) can now read:
  - h->mb_inspect_coeffs  = pre-IDCT coefficients (this patch)
  - h->cur_pic.f->data    = post-pixel-work pre-deblock reconstruction

and derive P = pixels − IDCT(C) for daedalus-decoder's frame-major
dispatch in PR-A3+.

Memcpy gated on (h->mb_inspect_cb != NULL).  Zero cost when no
consumer is registered.  Side buffer = 16 * 48 int16 = 1536 bytes
(matches the 8-bit half of sl->mb's int16_t[16 * 48 * 2] declared
size; high-bit-depth uses the upper half — not preserved here since
the daedalus-decoder consumer is 8-bit-only).

Single-threaded decode assumed at the consumer side
(avctx->thread_count = 1).  Multi-slice / multi-threaded streams
would race on the single side buffer — explicit limitation of the
inspection mechanism, future extension would put per-slice buffers
in H264SliceContext.

Verified: patches 0016 + 0017 apply cleanly and build in sequence
against the Kwiboo v4l2-request-n8.1 fork at the pinned commit
b57fbbe5.  ff_h264_set_mb_inspect_cb symbol exported as before.

Wired into arch PKGBUILD + debian build-deb.sh patch sequence.
pkgrel bumped 13 → 14.

Refs reauktion/daedalus-decoder!14 (PR-A2 callback wiring complete,
PR-A3 coefficient extraction is the next consumer).

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:${FFMPEG_VERSION}+rfourier+gb57fbbe
-PKGREL=13  # pkgrel=13 — per-MB inspection callback (0016) for daedalus-decoder CLI test harness; observation-only, no behaviour change to existing decode path
+PKGREL=14  # pkgrel=14 — per-MB coefficient side buffer (0017) extending 0016 for daedalus-decoder CLI IDCT validation; observation-only, no behaviour change to existing decode path
            # (cycle 9 of the daedalus-v4l2#11 step 2 substitution arc; closes
            # the libavcodec.so substitution sequence 6 IDCT4 / 7 IDCT8 /
            # 8 luma-v deblock / 9 qpel mc20).  Pulls daedalus-fourier PR #2
@@ -83,6 +83,7 @@ patch -Np1 -i "$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 "$HERE/0016-h264-mb-inspect-callback.patch"
+patch -Np1 -i "$HERE/0017-h264-mb-coeffs-side-buffer.patch"
 
 # --- daedalus-fourier: fetch + build static .a with PIC, install to a
 # per-build prefix; libavcodec.so links it into the shared object so