ffmpeg-v4l2-request-fourier: substitute H.264 IDCT 8×8 → daedalus-fourier

Cycle 7 of the libavcodec.so substitution arc (reauktion/daedalus-v4l2#11
step 2).  H264DSPContext.idct8_add — called per 8×8 block from the
High-profile intra-8×8-DCT decode path in libavcodec/h264_mb.c — now
dispatches through daedalus_recipe_dispatch_h264_idct8 instead of
ff_h264_idct8_add_neon.

## What

- Add 0004-h264-idct8-daedalus-fourier.patch (in both arch/ and debian/
  ffmpeg-v4l2-request-fourier/).  Extends libavcodec/aarch64/
  h264_idct_daedalus.c (introduced by 0003) with ff_h264_idct8_add_daedalus
  and a daedalus_recipe_dispatch_h264_idct8 call; patches
  libavcodec/aarch64/h264dsp_init_aarch64.c to wire c->idct8_add to
  the new shim.
- arch/PKGBUILD + debian/build-deb.sh: append the new patch to the
  apply list; bump pkgrel/PKGREL to 7.
- No new build-deps, no Depends change, no daedalus-fourier rev — the
  d87239d pin already exposes daedalus_recipe_dispatch_h264_idct8.

## Why

The recipe layer picks the substrate; for cycle 7 (H.264 IDCT 8×8)
the recipe is CPU NEON, so this is effectively a NEON-to-NEON
substitution layered on top of cycle 6.  Production validation of
cycle 6 on higgs Firefox YouTube: 3040 frames decoded cleanly,
avg_decode_us=3388 (no regression vs the pre-substitution ~4 ms
baseline).  Cycle 7 inherits the same shim's pthread_once context.

Bit-exact against ff_h264_idct8_add_neon (daedalus-fourier cycle 7
green; FFmpeg 8×8 block storage block[r + 8*c] matches daedalus
column-major convention).

## Scope NOT covered (deferred)

- Bulk c->idct8_add4 (inter 8×8-DCT macroblocks) stays on the
  in-tree NEON .S code; batched substitution with n_blocks>1 lands
  later alongside the cycle-6 bulk-paths work.
- High-bit-depth (10-bit) path untouched.
- Cycles 8/9 — separate PRs.

## SONAME

Unchanged.  libavcodec.so.62 / libavformat.so.62 / libavutil.so.60.

## Refs

- reauktion/daedalus-v4l2 issue #11 (substitution arc): reauktion/daedalus-v4l2#11
- marfrit-packages PR #76 (cycle 6 IDCT 4×4)
- marfrit-packages PR #78 (libxml2 ABI-skew workaround)
- marfrit/daedalus-fourier cycle 7 close (H.264 IDCT 8×8 NEON green)

debian/ffmpeg-v4l2-request-fourier/build-deb.sh

@@ -33,13 +33,11 @@ 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=6  # pkgrel=6 — drop --enable-libxml2 to avoid runner/target libxml2
-          # SOVERSION skew (runner has libxml2 ≥ 2.14 = SONAME 16; trixie
-          # has 2.12 = SONAME 2; -5 .deb dlopens libavformat → fails on
-          # "libxml2.so.16: cannot open shared object").  Neither the
-          # daedalus-v4l2 daemon (direct AVPacket feed) nor mpv-fourier
-          # nor firefox-fourier consumers need FFmpeg's DASH demuxer.
-          # (2026-05-21)
+PKGREL=7  # pkgrel=7 — H.264 IDCT 8x8 daedalus-fourier substitution
+          # (cycle 7).  Stacks on top of cycle-6 IDCT 4x4 (PR #76) and
+          # the libxml2-drop ABI-skew workaround (PR #78).  Wires
+          # H264DSPContext.idct8_add through
+          # daedalus_recipe_dispatch_h264_idct8.  (2026-05-22)
 
 # daedalus-fourier pin — first kernel substitution in libavcodec (cycle 6
 # H.264 IDCT 4x4).  Same SHA as the daedalus-v4l2 daemon already ships
@@ -69,6 +67,7 @@ fi
 patch -Np1 -i "$HERE/0001-libudev-bypass-fallback.patch"
 patch -Np1 -i "$HERE/0002-nv15-to-p010-unpack.patch"
 patch -Np1 -i "$HERE/0003-h264-idct4-daedalus-fourier.patch"
+patch -Np1 -i "$HERE/0004-h264-idct8-daedalus-fourier.patch"
 
 # --- daedalus-fourier: fetch + build static .a with PIC, install to a
 # per-build prefix; libavcodec.so links it into the shared object so