feat(ci): active monitoring for Netflix#1494 upstream PR (ADR-0448)

[lusoris]

Summary

Per the 2026-05-15 "no silent deferrals" rule: every `docs/state.md` row that names an external trigger needs an in-tree watcher. First instance: a weekly cron polling Netflix#1494 + opens a fork-side tracking issue when it merges. Closes the failure mode where the previous "scheduled remote agent re-runs weekly" comment had no in-tree audit trail.

Opened as draft per the 1-active-PR train discipline.

Type

• `feat` / `ci` — governance + monitoring infrastructure

Reproducer

```bash
gh workflow run upstream-netflix-955-watcher.yml --repo lusoris/vmaf
gh run list --workflow upstream-netflix-955-watcher.yml --repo lusoris/vmaf --limit 1

Expect: workflow runs, no fork-side issue opened (Netflix#1494 still OPEN

as of 2026-05-15).

```

Bug-status hygiene

• no state delta in this PR; the workflow MAKES the existing `Weird rounding in i4_adm_cm Netflix/vmaf#955` row actively-monitored. A follow-up doc-only PR can add a "watched by" cross-link.

Netflix golden-data gate

• No `python/test/*.py` `assertAlmostEqual` value modified.

Cross-backend numerical results

No cross-backend impact: CI infrastructure only.

Performance

Negligible: ~5 s per weekly run × 1 deferral × 52 weeks ≈ 4 minutes/year of GitHub Actions time.

Deep-dive deliverables (ADR-0108)

• Research digest — the 2026-05-15 user direction itself + audit slice E finding that Weird rounding in i4_adm_cm Netflix/vmaf#955 had been deferred since 2026-04-25 with no in-tree verification of the external scheduler firing.
• Decision matrix — `docs/adr/0448-active-upstream-monitoring-discipline.md` § "Alternatives considered" enumerates 5 options.
• AGENTS.md invariant note — `.github/workflows/` is fork-local infrastructure; the watcher naming convention (`upstream--watcher.yml`) is documented in ADR-0448.
• Reproducer / smoke-test command — see Reproducer block above.
• CHANGELOG fragment — `changelog.d/added/upstream-netflix-955-watcher.md`.
• Rebase note — fork-local workflow + ADR; no upstream-mirror-touched files.

Known follow-ups

1. Same shape for the other 4 external-trigger deferrals: HDR-VMAF-MODEL-PORT, T-FFMPEG-HIP-FILTER-DEFERRED, Upstream-port-later batch.
2. Add a "watched by" cross-link to each existing state.md row pointing at its watcher workflow file.

Breaking changes

None.

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