port(libvmaf/feature/x86): AVX2/AVX-512 portability fixes (Netflix PR #1475)

[lusoris]

Summary

Cherry-picks Netflix PR #1475
("AVX2: replace non-portable vector indexing & SIMD casts with intrinsic
extraction/reinterpretation") onto fork master.

Two upstream commits land:

• c437f43d — replaces GCC vector subscripting (rN[0] + rN[1]) with
  _mm_extract_epi64 / _mm256_extract_epi64 across adm_avx2.c,
  adm_avx512.c, motion_avx2.c, motion_avx512.c. The vector-subscript
  syntax is a GCC extension; MSVC and stricter compilers reject it.
• b4fcd21b — replaces a stray C-style (__m256i) cast on a __m256
  operand in adm_avx2.c with _mm256_castps_si256.

The third upstream commit (50e772a1, cosmetic indentation fix)
collapsed to a no-op against our existing clang-format-22 wrapped
formatting and was skipped.

10 conflicts arose from the fork's own modifications on these same files
(rebase-notes entries 0001 and 0013); all resolved by keeping our
wrapped formatting and adopting upstream's portable intrinsic
substitutions.

Type

• port — cherry-pick from upstream Netflix/vmaf

Checklist

• Commits follow Conventional Commits (PR will squash to a single
  port(...) subject; the two cherry-picks preserve upstream attribution
  to David C. Manuelda).
• Unit tests pass: meson test -C libvmaf/build → 27/27 OK.
• SIMD-only port; no GPU paths touched. Cross-backend ULP unchanged.
• No new files; no license-header concerns.

Netflix golden-data gate

• Did not modify any assertAlmostEqual(...) score in the
  Netflix golden Python tests.
• Netflix golden VMAF mean unchanged from entry 0013 baseline
  (76.66783).

Deep-dive deliverables

• Research digest — no digest needed: trivial portability
  substitution; intrinsics-equivalence is documented by Intel.
• Decision matrix — no alternatives: only-one-way fix
  (substitute the documented portable intrinsic for the GCC extension).
• AGENTS.md invariant note — covered by the existing
  libvmaf/src/feature/x86/AGENTS.md direction "match upstream byte-for-byte
  modulo our wrapped formatting"; the new invariant is captured in
  rebase-note 0016.
• Reproducer / smoke-test command — see below.
• CHANGELOG.md "lusoris fork" entry — added under
  ### Changed ("Upstream port -- AVX2/AVX-512 portability").
• Rebase note — docs/rebase-notes.md entry 0016 (numbering may
  shift to 0014/0015/0017 depending on PR-fix(ci): coverage gate lcov→gcovr + ORT + lint upstream tests in-tree #46 / PR-fix(libvmaf/feature): free VIF init base pointer on fail path (Netflix PR #1476, leak half) #47 merge order; will
  rebase if needed).

Reproducer

```bash
ninja -C libvmaf/build && meson test -C libvmaf/build
libvmaf/build/tools/vmaf -r python/test/resource/yuv/src01_hrc00_576x324.yuv \
-d python/test/resource/yuv/src01_hrc01_576x324.yuv \
-w 576 -h 324 -p 420 -b 8 \
--model version=vmaf_v0.6.1 -o /tmp/vmaf-1475-port.json
grep -E '<metric name="vmaf"' /tmp/vmaf-1475-port.json

Expected: vmaf mean unchanged from entry 0013 (~ 76.66783).

```

Known follow-ups

• The cosmetic upstream commit 50e772a1 was skipped as a no-op against
  our clang-format-22 pass; re-running upstream sync should not surface
  it again.

🤖 Generated with Claude Code

[lusoris]

Closing as fully superseded by the MSVC Windows portability sweep already on master (rebase-notes 0022, commits ddb58bdb, 555de409, and siblings).

Verified by git diff origin/master..port/upstream-pr-1475-avx2-portability -- libvmaf/src/feature/x86/:

• Master already has the _mm256_castps_si256(_mm256_cmp_ps(...)) migration in adm_avx2.c, adm_avx512.c, motion_avx2.c, motion_avx512.c.
• Master already has _mm_extract_epi64() replacing GCC vector-indexing in the same files.
• Master additionally has refinements this PR does not carry: explicit (int64_t) / (uint64_t) casts on the extract-and-sum expressions, #include "feature/compat_builtin.h", and an explanatory comment about the GNU vector-extension cast that MSVC rejects.

Merging this PR would revert those refinements. Closing in favour of the master state. No code intent from upstream Netflix PR Netflix#1475 is lost.