ffmpeg-v4l2-request-fourier: restore AV_CODEC_FLAG_LOW_DELAY in H.264 decoder

FFmpeg 8.x dropped the H.264 decoder's low_delay code path —
AV_CODEC_FLAG_LOW_DELAY no longer prevents h264_select_output_frame
from running the display-order DPB output queue.  The daedalus-v4l2
daemon's `ctx->flags |= AV_CODEC_FLAG_LOW_DELAY` at
daemon/src/decoder.c:202 has been a silent no-op since the SONAME
61→62 jump landed in reauktion/daedalus-v4l2 PR #16; on Firefox
YouTube this re-introduced the 2-1-4-3 B-frame pair-swap that PR
#12's daemon flag was supposed to prevent.

Fix lives in libavcodec, not the daemon: restore the documented
LOW_DELAY semantics so the daemon (and any other V4L2-stateless-
style consumer) keeps the one-frame-per-send_packet decode-order
output contract it already declares.

## Patch

0006-h264-restore-low-delay.patch touches libavcodec/h264_slice.c:

- h264_select_output_frame: early-exit when LOW_DELAY is set.
  Emit the just-decoded picture as next_output_pic, mirror the
  corruption / recovery-point tracking the main path performs,
  skip delayed_pic[] / POC reorder machinery entirely.

- h264_field_start: suppress the SPS-driven
  `has_b_frames = sps->num_reorder_frames` clobber when LOW_DELAY
  is set.  Without this the per-slice bitstream_restriction_flag
  re-pickup would reintroduce a nonzero reorder buffer mid-stream
  even after the daemon set has_b_frames=0 at avcodec_open2.

## Why not daemon-side

A daemon SPS-rewrite (`num_reorder_frames=0`) was considered but
rejected: it works only for the daemon's reconstructed SPS NAL,
not for any in-band SPS the daemon dlopens libavformat to parse
in other code paths.  Restoring documented FFmpeg flag semantics
is the smaller, more durable change and keeps the daemon
interface stable.

## Packaging

- PKGREL/pkgrel bump to 9.
- No new build-deps, no Depends change.
- Substitution arc cycles 6/7/8 unchanged.

## Refs

- reauktion/daedalus-v4l2#11 / #12 (LOW_DELAY half-measure on
  daemon side, originally landed against FFmpeg 7.x).
- daemon/src/decoder.c:202 (`ctx->flags |= AV_CODEC_FLAG_LOW_DELAY`
  for H.264 only — unchanged, but now actually has effect again).

arch/ffmpeg-v4l2-request-fourier/PKGBUILD

@@ -24,7 +24,7 @@ _srcname=FFmpeg
 _version='8.1'
 _commit='b57fbbe50c9b2656fad86a1a7eeabfd2b2a50935'  # v4l2-request-n8.1 tip 2026-04-24
 pkgver=8.1.r123329.b57fbbe
-pkgrel=8   # pkgrel=8 — H.264 luma-v deblock daedalus-fourier substitution (cycle 8, 2026-05-22)
+pkgrel=9   # pkgrel=9 — restore AV_CODEC_FLAG_LOW_DELAY for H.264 (2026-05-22)
 epoch=2
 
 # daedalus-fourier pin — first kernel substitution in libavcodec
@@ -92,8 +92,9 @@ source=("git+https://github.com/Kwiboo/FFmpeg.git#commit=${_commit}"
         '0002-nv15-to-p010-unpack.patch'
         '0003-h264-idct4-daedalus-fourier.patch'
         '0004-h264-idct8-daedalus-fourier.patch'
-        '0005-h264-deblock-luma-v-daedalus-fourier.patch')
-sha256sums=('SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP')
+        '0005-h264-deblock-luma-v-daedalus-fourier.patch'
+        '0006-h264-restore-low-delay.patch')
+sha256sums=('SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP')
 
 pkgver() {
   cd "${_srcname}"
@@ -109,6 +110,7 @@ prepare() {
   patch -Np1 -i "${srcdir}/0003-h264-idct4-daedalus-fourier.patch"
   patch -Np1 -i "${srcdir}/0004-h264-idct8-daedalus-fourier.patch"
   patch -Np1 -i "${srcdir}/0005-h264-deblock-luma-v-daedalus-fourier.patch"
+  patch -Np1 -i "${srcdir}/0006-h264-restore-low-delay.patch"
 }
 
 build() {