ffmpeg-v4l2-request-fourier: substitute H.264 IDCT 4×4 → daedalus-fourier

First cycle of the libavcodec.so substitution arc (reauktion/daedalus-v4l2#11
step 2).  H264DSPContext.idct_add — called per 4×4 block from the
intra-4×4 decode path in libavcodec/h264_mb.c — now dispatches through
daedalus_recipe_dispatch_h264_idct4 instead of ff_h264_idct_add_neon.

## What

- Add 0003-h264-idct4-daedalus-fourier.patch (in both arch/ and
  debian/ ffmpeg-v4l2-request-fourier/).  Creates
  libavcodec/aarch64/h264_idct_daedalus.c (ff_h264_idct_add_daedalus
  shim + lazy pthread_once context init via
  daedalus_ctx_create_no_qpu), patches
  libavcodec/aarch64/h264dsp_init_aarch64.c to wire c->idct_add to
  the shim, adds the new .o to libavcodec/aarch64/Makefile.
- arch/PKGBUILD + debian/build-deb.sh: fetch + build
  daedalus-fourier (pinned at d87239d — lockstep with the
  daedalus-v4l2 daemon's inline build) with
  -DCMAKE_POSITION_INDEPENDENT_CODE=ON into a per-build temp prefix,
  then pass --extra-cflags=-I.../include --extra-ldflags=-L.../lib
  --extra-libs="-ldaedalus_core -lvulkan -lpthread" to FFmpeg
  configure.  daedalus_core.a is static-linked into libavcodec.so.62.
- debian/control Depends gains libvulkan1 (daedalus_core PUBLIC-links
  Vulkan::Vulkan for the queryable QPU substrate; the no-QPU
  constructor still works at runtime but the loader needs
  libvulkan.so.1 present to dlopen libavcodec.so.62).
- arch/PKGBUILD depends gains vulkan-icd-loader, makedepends gains
  cmake / ninja / vulkan-headers.

## Why

The recipe layer picks the substrate; for cycle 6 (H.264 IDCT 4×4)
the recipe is CPU NEON, so this is effectively a NEON-to-NEON
substitution with one extra dispatch call and recipe-table lookup.
The point of this first cycle isn't perf wins — it's plumbing.  Once
the path is wired and stable, follow-up patches batch through the
bulk paths (idct_add16 / idct_add16intra / idct_add8) and stack
cycles 7/8/9 (IDCT 8×8, luma-v deblock, qpel mc20).

Bit-exact against ff_h264_idct_add_neon (daedalus-fourier cycle 6
green; FFmpeg's 4×4 block storage matches daedalus's column-major
convention).

## Scope NOT covered

- Bulk paths (idct_add16 / idct_add16intra / idct_add8) — most IDCT
  4×4 calls in real H.264 streams go through these, not the per-
  block c->idct_add path; intra-4×4-only macroblocks are a minority.
  Batched substitution lands in a follow-up.
- High-bit-depth (10-bit) path — not touched; 8-bit only.
- Cycles 7/8/9 — separate PRs.

## SONAME

Unchanged.  libavcodec.so.62 / libavformat.so.62 / libavutil.so.60.
No daedalus-v4l2-dkms or daedalus-v4l2 bump required.

## Refs

- reauktion/daedalus-v4l2 issue #11 (substitution arc): reauktion/daedalus-v4l2#11
- marfrit/daedalus-fourier cycle 6 close (H.264 IDCT 4×4 NEON green)

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

@@ -33,7 +33,12 @@ 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=2  # pkgrel=2 — Path A move to /opt/fourier prefix (2026-05-19)
+PKGREL=3  # pkgrel=3 — H.264 IDCT 4x4 daedalus-fourier substitution (2026-05-21)
+
+# daedalus-fourier pin — first kernel substitution in libavcodec (cycle 6
+# H.264 IDCT 4x4).  Same SHA as the daedalus-v4l2 daemon already ships
+# inline; rev in lockstep with the daemon when the public API rolls.
+DAEDALUS_FOURIER_COMMIT=d87239d8172307d9a1b93c95cbed116d175b85cc
 
 HERE=$(dirname "$(readlink -f "$0")")
 
@@ -57,6 +62,34 @@ fi
 # Apply patches (same as Arch).
 patch -Np1 -i "$HERE/0001-libudev-bypass-fallback.patch"
 patch -Np1 -i "$HERE/0002-nv15-to-p010-unpack.patch"
+patch -Np1 -i "$HERE/0003-h264-idct4-daedalus-fourier.patch"
+
+# --- daedalus-fourier: fetch + build static .a with PIC, install to a
+# per-build prefix; libavcodec.so links it into the shared object so
+# H264DSPContext.idct_add (and follow-up kernels) dispatch through the
+# daedalus recipe layer instead of the in-tree NEON .S code. ---
+#
+# PIC is mandatory — the static .a is linked into a .so, so all object
+# code must be relocatable.  Vulkan is PUBLIC-linked by daedalus_core
+# (queryable QPU substrate); we add libvulkan1 to Debian Depends below
+# so dlopen of libavcodec.so.62 succeeds on stock trixie.
+FOURIER_PREFIX=$work/fourier-prefix
+mkdir -p "$FOURIER_PREFIX"
+
+pushd "$work" >/dev/null
+curl --connect-timeout 10 --max-time 600 --retry 3 --retry-delay 5 -sSLfo daedalus-fourier.tar.gz \
+    "https://git.reauktion.de/marfrit/daedalus-fourier/archive/${DAEDALUS_FOURIER_COMMIT}.tar.gz"
+tar xzf daedalus-fourier.tar.gz
+pushd daedalus-fourier >/dev/null
+cmake -B build -G Ninja \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_POSITION_INDEPENDENT_CODE=ON \
+    -DCMAKE_INSTALL_PREFIX="$FOURIER_PREFIX"
+cmake --build build --target daedalus_core
+cmake --install build
+popd >/dev/null
+popd >/dev/null
+cd "$work/FFmpeg"
 
 # Configure with Arch-parity flags.  Drops the same set of features
 # (X11, AMF, CUDA, FireWire, AviSynth, Bluray, OpenMPT, JPEG-XL,
@@ -73,6 +106,9 @@ patch -Np1 -i "$HERE/0002-nv15-to-p010-unpack.patch"
     --mandir=/opt/fourier/share/man \
     --extra-ldexeflags='-Wl,-rpath,/opt/fourier/lib' \
     --extra-ldsoflags='-Wl,-rpath,/opt/fourier/lib' \
+    --extra-cflags="-I${FOURIER_PREFIX}/include" \
+    --extra-ldflags="-L${FOURIER_PREFIX}/lib" \
+    --extra-libs="-ldaedalus_core -lvulkan -lpthread" \
     --disable-debug \
     --disable-static \
     --disable-doc \
@@ -147,6 +183,7 @@ Priority: optional
 Architecture: arm64
 Depends: libc6,
          libdrm2,
+         libvulkan1,
          libfontconfig1,
          libfreetype6,
          libfribidi0,