Cycle 8 (H.264 deblock) opened — Phase 1 + NEON vendored

Targets the one H.264 kernel most likely to be QPU-worthy:
in-loop deblock. Cycles 6 and 7 (IDCT 4x4 and 8x8) both came in
CPU-only because H.264 transforms are NEON-trivial. H.264
deblock has analogous structure to VP9 LPF (cycles 2+4, both
GREEN) so predicted R8 = ORANGE/YELLOW.

This commit:
- Vendors ff_h264_*_loop_filter_*_neon from h264dsp_neon.S
  (1076 lines, includes both v/h luma + chroma + intra variants
  + weight/biweight)
- PROVENANCE.md updated with the new vendored file
- Phase 1 doc captures the full plan: start with luma vertical
  non-intra (most common case), defer Phase 3+ to next session

H.264 deblock C ref scope is ~2 hours (per-row branching,
per-4-row-segment tc0, ap/aq side conditions, alpha/beta
thresholds — much more complex than VP9 LPF wd=4's
single-branch filter). Deferring to fresh attention next
session rather than rushing now.

After cycle 8 closes, the H.264 QPU surface is well-characterised
and the cycles-1-8 inventory drives the Phase 8 V4L2 wrapper's
substrate-routing recipe.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

external/ffmpeg-snapshot/PROVENANCE.md

@@ -27,6 +27,7 @@ tagged commit, no modifications.
 | `libavcodec/aarch64/vp9lpf_neon.S` | 1334 | — | `384e49e7a6e838d9e38aedc00838ed4aebfa6c5bdb343ecaf23ef639bc10fbb7` |
 | `libavcodec/aarch64/vp9mc_neon.S` | 665 | — | `6b1d50f9821742584fdd47758057f810644aff3a008faaa774ff5b9cac4d1fef` |
 | `libavcodec/aarch64/h264idct_neon.S` | 415 | 16269 | `963ffe5f31b5a6a422e13b0d394cf5630126927abfb23aa214f7cbe83d60683f` — H.264 IDCT 4×4/8×8/DC NEON kernels for cycle 6+ |
+| `libavcodec/aarch64/h264dsp_neon.S` | 1076 | — | `978e076f0020e688b40c6dd827708c3d53e17c64a99fd0052e43d983536ce638` — H.264 in-loop deblock + weight/biweight kernels for cycle 8+ |
 | `libavcodec/vp9_subpel_filters_table.c` | — | — | hand-extracted from `libavcodec/vp9dsp.c` at same n7.1.3 pin — provides `ff_vp9_subpel_filters` for `vp9mc_neon.S` to link against without dragging in vp9dsp.c's full init machinery |
 | `libavcodec/aarch64/neon.S` | 173 | 7496 | `72d36ce6c3fcc5e53de869cfe10fda16225ebe580c32891bccc240a30a85a538` |
 | `libavutil/aarch64/asm.S` | 260 | 8069 | `c0d03143b1bc5a9e358222d08d2d449d595271844fe7a3dc23bffb91abe8b0e3` |