docs(rebase-notes): Netflix#1486 motion updates verified present (ADR-0158)

[lusoris]

Summary

Verification pass confirmed that Netflix upstream PR #1486 "Port motion updates" (MERGED 2026-04-20) is already fully absorbed into the fork's master — not via a direct port PR but via earlier incremental motion3 / blend / five-frame-window / moving-average commits that landed separately.

Markers confirmed present in fork master:

• edge_8 mirror fix i_tap = height - (i_tap - height + 2) at integer_motion.c:240, x86/motion_avx2.c:147, x86/motion_avx512.c:147.
• motion_max_val option (default 10000.0, FEATURE_PARAM flag) at integer_motion.c:57,118-120.
• VMAF_integer_feature_motion3_score output plumbing in integer_motion.c + alias.c.
• All 73 coordinated Netflix golden assertAlmostEqual updates from upstream commit 62f47d5 present in fork's python test files.

Fork PR #45 (9371a0aa feat(libvmaf/feature): port upstream motion updates) tracked the 1:1 port attempt but never merged — its substance landed via fork-native work instead. End state is equivalent.

Scope

Doc-only close per user popup 2026-04-24 → "Doc-only PR":

• New docs/adr/0158-netflix-1486-motion-updates-verified-present.md — documents the coverage + invariants future /sync-upstream runs must preserve.
• docs/rebase-notes.md — entry 0051 lists the four markers + fork-local motion extensions that must not collide on rebase.
• docs/adr/README.md — ADR-0158 index row.
• BACKLOG T4-1 marked DONE (gitignored, not in PR diff).

No CHANGELOG entry — no user-visible delta.
No code change — meson test -C build → 35/35 (unchanged).

Test

• `git status` after docs changes → only the 3 doc files modified.
• `meson test -C build` → 35/35 pass (no delta vs master).
• Pre-commit hooks green.

Type

• `docs` — documentation

Netflix golden-data gate (ADR-0024)

• Did not modify any `assertAlmostEqual` value. The 73 upstream-coordinated updates were already present in fork master before this PR.

Cross-backend numerical results

```text
N/A — zero code change; no numerical path touched.
```

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: verification of upstream PR presence is not a research question.
• Decision matrix — ADR-0158 §Alternatives considered.
• `AGENTS.md` invariant note — no rebase-sensitive invariants: this ADR exists precisely to note that Port motion updates Netflix/vmaf#1486 is absorbed, so no invariant survives that a rebase could drop.
• Reproducer / smoke-test command — pasted below.
• `CHANGELOG.md` "lusoris fork" entry — no changelog needed: no user-visible change.
• Rebase note — entry 0051 in docs/rebase-notes.md.

Reproducer

```bash
grep -n "height - (i_tap - height + 2)|motion_max_val|VMAF_integer_feature_motion3_score" \
libvmaf/src/feature/integer_motion.c \
libvmaf/src/feature/alias.c \
libvmaf/src/feature/x86/motion_avx2.c \
libvmaf/src/feature/x86/motion_avx512.c

Expect: matches at all 4 files (verified locally).

ninja -C build && meson test -C build

Expect: 35/35 pass.

```

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