fix(ci): trigger Rust CI on libvmaf public header changes

[lusoris]

Summary

Under ADR-0313 a check that does not report on a pull request counts as passed, which turned .github/workflows/rust-ci.yml's too-narrow paths: filter into a silent bypass. bindings/rust/vmafx-sys/build.rs compiles C headers from core/include/libvmaf/libvmaf.h via bindgen, but rust-ci.yml previously filtered only on bindings/rust/**, core/src/feature/rust/**, Cargo.*, and deny.toml. This PR adds core/include/libvmaf/** to workflow path filters and CI impact selectors, accompanied by workflow contract and impact routing regression tests.

Type

• feat — new feature
• fix — bug fix
• perf — performance improvement
• refactor — no behavior change
• docs — documentation only
• test — test-only
• build / ci — tooling / infra
• port — cherry-pick from upstream Netflix/vmaf
• sycl / cuda / simd — backend-specific

Checklist

• Commits follow Conventional Commits (the commit-msg hook enforces this).
• make format && make lint is green locally.
• Unit tests pass: meson test -C build.
• If I touched any SIMD/GPU code path, I ran /cross-backend-diff and the worst ULP is ≤ 2.
• If I touched a feature extractor with SIMD/GPU twins, I either updated every twin or listed the gap under "Known follow-ups" below.
• If I added a new .c / .cpp / .cu / .h / .hpp, it has the appropriate license header (see CONTRIBUTING.md).
• If this is a breaking change, the commit message uses ! or BREAKING CHANGE: and the migration path is documented below.
• If this PR adds an ADR, the ADR row lives in docs/adr/_index_fragments/<NNNN-slug>.md and the slug is appended to docs/adr/_index_fragments/_order.txt — do not edit docs/adr/README.md directly (regenerated by scripts/docs/concat-adr-index.sh; see ADR-0221).

Bug-status hygiene (ADR-0165)

• docs/state.md updated in this PR with a row
  in the appropriate section (Open / Recently closed / Confirmed
  not-affected / Deferred), OR no state delta: REASON.

Netflix golden-data gate (ADR-0024)

• I did not modify any assertAlmostEqual(...) score in the Netflix golden Python tests.
• If I believe a golden value must change, I have explained why below AND pinged @lusoris for a CODEOWNERS exception.

Cross-backend numerical results

N/A - CI and workflow path filtering defect.

Deep-dive deliverables (ADR-0108)

• Research digest — docs/research/2080-rust-ci-path-filter-coverage.md written and linked.
• Decision matrix — captured in docs/research/2080-rust-ci-path-filter-coverage.md §3.
• AGENTS.md invariant note — added to bindings/rust/vmafx-sys/AGENTS.md.
• Reproducer / smoke-test command — pasted below under "Reproducer".
• CHANGELOG fragment — changelog.d/fixed/rust-ci-path-filter-libvmaf-headers.md.
• Rebase note — entry added to docs/rebase-notes.md under fix/rust-ci-path-filters.

Reproducer

# Proves red before fix:
python3 -m unittest scripts/ci/tests/test_rust_ci_workflow_contract.py
python3 -m unittest scripts/ci/tests/test_ci_impact.py -k test_libvmaf_public_header_change_selects_rust_and_c_core

Known follow-ups

This PR resolves only the confirmed Rust CI blind spot. Canonical BUG-098 remains open until the residual path-filter audit covers FFmpeg, container, Doxygen, and Intel LLVM workflows.

[lusoris]

Superseded by #1561 (squash dd51d00). Instead of adding core/include/libvmaf/** to the Rust workflow's path filter, the train removed workflow-level path filters from all seven affected workflows; rust-ci.yml now runs on every PR through the ADR-1140 impact planner, and .github/ci-impact.json routes core/include/libvmaf/** to the Rust lane. docs/state.md records BUG-098 / T-PATH-FILTERS-WEAKEN-NEW-GATES-2026-09-22 as fixed. This branch's commit 9f72ec3 is therefore obsolete, not lost.