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)

arch/ffmpeg-v4l2-request-fourier/PKGBUILD

@@ -24,8 +24,13 @@ _srcname=FFmpeg
 _version='8.1'
 _commit='b57fbbe50c9b2656fad86a1a7eeabfd2b2a50935'  # v4l2-request-n8.1 tip 2026-04-24
 pkgver=8.1.r123329.b57fbbe
-pkgrel=5
+pkgrel=6   # pkgrel=6 — H.264 IDCT 4x4 daedalus-fourier substitution (2026-05-21)
 epoch=2
+
+# daedalus-fourier pin — first kernel substitution in libavcodec
+# (cycle 6 H.264 IDCT 4x4).  Same SHA as the daedalus-v4l2 daemon's
+# inline build; lockstep with that until the public API rolls.
+_daedalus_fourier_commit='d87239d8172307d9a1b93c95cbed116d175b85cc'
 pkgdesc='FFmpeg with V4L2 Request API hwaccel (Rockchip / Allwinner stateless decode)'
 arch=('aarch64')
 url='https://github.com/Kwiboo/FFmpeg'
@@ -34,6 +39,7 @@ depends=(
   alsa-lib
   bzip2
   fontconfig
+  vulkan-icd-loader
   fribidi
   gmp
   gnutls
@@ -59,10 +65,13 @@ depends=(
   zlib
 )
 makedepends=(
+  cmake
   git
   linux-api-headers
   mesa
   nasm
+  ninja
+  vulkan-headers
 )
 provides=(
   libavcodec.so
@@ -78,9 +87,11 @@ provides=(
 conflicts=(ffmpeg)
 replaces=(ffmpeg ffmpeg-v4l2-request-git)
 source=("git+https://github.com/Kwiboo/FFmpeg.git#commit=${_commit}"
+        "daedalus-fourier-${_daedalus_fourier_commit}.tar.gz::https://git.reauktion.de/marfrit/daedalus-fourier/archive/${_daedalus_fourier_commit}.tar.gz"
         '0001-libudev-bypass-fallback.patch'
-        '0002-nv15-to-p010-unpack.patch')
-sha256sums=('SKIP' 'SKIP' 'SKIP')
+        '0002-nv15-to-p010-unpack.patch'
+        '0003-h264-idct4-daedalus-fourier.patch')
+sha256sums=('SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP')
 
 pkgver() {
   cd "${_srcname}"
@@ -93,9 +104,25 @@ prepare() {
   cd "${_srcname}"
   patch -Np1 -i "${srcdir}/0001-libudev-bypass-fallback.patch"
   patch -Np1 -i "${srcdir}/0002-nv15-to-p010-unpack.patch"
+  patch -Np1 -i "${srcdir}/0003-h264-idct4-daedalus-fourier.patch"
 }
 
 build() {
+  # --- daedalus-fourier: 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. ---
+  local _fourier_prefix="${srcdir}/fourier-prefix"
+  mkdir -p "${_fourier_prefix}"
+  pushd "${srcdir}"/daedalus-fourier >/dev/null
+  cmake -B build -G Ninja \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_POSITION_INDEPENDENT_CODE=ON \
+    -DCMAKE_INSTALL_PREFIX="${_fourier_prefix}"
+  cmake --build build --target daedalus_core
+  cmake --install build
+  popd >/dev/null
+
   cd "${_srcname}"
 
   # FFmpeg's configure resolves the compiler via `which` and bakes the
@@ -147,6 +174,9 @@ build() {
     --enable-libx265 \
     --enable-libwebp \
     \
+    --extra-cflags="-I${_fourier_prefix}/include" \
+    --extra-ldflags="-L${_fourier_prefix}/lib" \
+    --extra-libs="-ldaedalus_core -lvulkan -lpthread" \
     --host-cflags='-fPIC'
 
   make