feat(hip): T7-10 — HIP (AMD) backend scaffold (audit-first)

[lusoris]

Summary

Mirrors the Vulkan T5-1 scaffold (ADR-0175)
for the HIP / AMD ROCm backend. Lands the static surfaces (header,
build wiring, kernel stubs, smoke test, CI matrix row, docs) so the
runtime + first-kernel PRs that follow have a stable base to land on.

• Public header libvmaf/include/libvmaf/libvmaf_hip.h declaring
  VmafHipState / VmafHipConfiguration / vmaf_hip_state_init /
  _import_state / _state_free / vmaf_hip_list_devices /
  vmaf_hip_available. All entry points return -ENOSYS until the
  runtime PR (T7-10b) lands.
• New libvmaf/src/hip/ (common.{c,h}, picture_hip.{c,h},
  dispatch_strategy.{c,h}, meson.build) + libvmaf/src/feature/hip/
  (3 kernel stubs: adm / vif / motion + feature_hip.h).
• New enable_hip boolean option (default false) wired through
  libvmaf/src/meson.build and the public-header install rule.
• New 9-sub-test smoke at libvmaf/test/test_hip_smoke.c exercising
  every public C-API entry point.
• New CI matrix row Build — Ubuntu HIP (T7-10 scaffold).
• New docs: docs/backends/hip/overview.md, ADR-0209,
  Research-0032; index + build-flags + rebase-notes + AGENTS.md +
  CHANGELOG updated.

Zero hard runtime dependencies — dependency('hip-lang', required: false)
is silently absent on stock Ubuntu runners; ROCm SDK arrives with the
runtime PR.

Deep-dive deliverables (ADR-0108)

• Research digest — docs/research/0032-hip-applicability.md (AMD market share + ROCm 6.x Linux maturity check; ADR-0175 Vulkan precedent does not generalize to the AMD-specific runtime cost question).
• Decision matrix — ADR-0209 § Alternatives considered (separate libvmaf_hip.so vs in-tree, AMD-only vs hipify auto-translation, ROCm vs HIP runtime, boolean vs feature option type, smoke vs no-smoke).
• AGENTS.md invariant note — libvmaf/AGENTS.md § "Rebase-sensitive invariants" gains the HIP scaffold contract row (5 sub-invariants pinning option type, ENOSYS contract, build-wiring placement, header purity).
• Reproducer / smoke-test command — see Test plan.
• CHANGELOG.md entry — Unreleased § Added.
• Rebase note — docs/rebase-notes.md new entry 0074 pinning the HIP-scaffold-vs-Netflix-upstream divergence.

Test plan

• CPU-only build still green:

  meson setup build-cpu libvmaf -Denable_cuda=false -Denable_sycl=false
  ninja -C build-cpu
  meson test -C build-cpu             # 37/37 pass
  

• HIP-enabled scaffold build green:

  meson setup build-hip libvmaf -Denable_hip=true -Denable_cuda=false -Denable_sycl=false
  ninja -C build-hip
  meson test -C build-hip             # 38/38 pass (37 + test_hip_smoke)
  meson test -C build-hip test_hip_smoke -v   # 9/9 sub-tests pass
  

• Touched files lint-clean (clang-format + clang-tidy + pre-commit + scripts/ci/check-copyright.sh + scripts/ci/assertion-density.sh).
• CI matrix row Build — Ubuntu HIP (T7-10 scaffold) will exercise the scaffold against bit-rot every PR.

What lands next

1. Runtime PR (T7-10b): real hipInit / hipGetDeviceCount / stream creation; vmaf_hip_state_init returns 0 on a real device.
2. VIF kernel PR (T7-10c): first feature on the HIP compute path (same pathfinder choice as Vulkan T5-1b).
3. ADM + motion + long-tail kernels.
4. CI ROCm runner — when GitHub Actions exposes AMD GPUs.

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