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/debian/changelog

@@ -1,3 +1,32 @@
+ffmpeg-v4l2-request-fourier (2:8.1+rfourier+gb57fbbe-3) bookworm trixie; urgency=medium
+
+  * Add 0003-h264-idct4-daedalus-fourier.patch — H264DSPContext.idct_add
+    (per-block 4x4 IDCT, called from the intra-4x4 decode path in
+    libavcodec/h264_mb.c) now dispatches through
+    daedalus_recipe_dispatch_h264_idct4 instead of
+    ff_h264_idct_add_neon.  First end-to-end exercise of the
+    daedalus-fourier kernel pack inside libavcodec.so on the
+    production decode hot path (daedalus-v4l2#11 step 2 — cycle 6
+    H.264 IDCT 4x4, NEON-by-recipe).
+  * build-deb.sh: fetches + builds daedalus-fourier (pinned at
+    d87239d, lockstep with the daemon's static link) with
+    -fPIC into a per-build temp prefix, then passes
+    --extra-cflags=-I.../include --extra-ldflags=-L.../lib
+    --extra-libs="-ldaedalus_core -lvulkan -lpthread" to FFmpeg
+    configure.  Static-linked into libavcodec.so.62.
+  * Bulk paths (idct_add16 / idct_add16intra / idct_add8) remain on
+    the stock NEON .S code and will be batched through
+    daedalus_recipe_dispatch_h264_idct4 with n_blocks>1 in a
+    follow-up.  Cycles 7/8/9 (IDCT 8x8 / luma-v deblock / qpel mc20)
+    land in subsequent patches.
+  * Depends gains libvulkan1 — daedalus_core PUBLIC-links Vulkan
+    (queryable QPU substrate); the no-QPU constructor still works,
+    but the loader refuses libavcodec.so.62 at dlopen time without
+    libvulkan.so.1 present.
+  * No ABI change; SONAMEs stay 62/62/60.
+
+ -- Markus Fritsche <mfritsche@reauktion.de>  Thu, 21 May 2026 20:00:00 +0000
+
 ffmpeg-v4l2-request-fourier (2:8.1+rfourier+gb57fbbe-1) bookworm trixie; urgency=medium
 
   * Initial Debian packaging for the Kwiboo FFmpeg fork with V4L2