refactor(vulkan): T-GPU-DEDUP-19 — vif_vulkan migrated to kernel_template + _add_variant

[lusoris]

Summary

Multi-pipeline-via-variant migration on top of PR #272 (which
adds the vmaf_vulkan_kernel_pipeline_add_variant() helper).

State collapses dsl + pipeline_layout + shader + desc_pool + pipelines[4] to VmafVulkanKernelPipeline pl + VkPipeline scale_variants[3].

• Scale 0 is the template's base pipeline.
• Scales 1, 2, 3 are sibling pipelines via _add_variant() —
  same layout, shader, DSL, pool, different SCALE
  spec-constant.

New vif_scale_pipeline() accessor maps scale index to the
right VkPipeline handle. close_fex destroys the 3 scale
variants first, then calls
vmaf_vulkan_kernel_pipeline_destroy() on the bundle (same
invariant as ssim_vulkan in T-GPU-DEDUP-7 and psnr_hvs_vulkan in
T-GPU-DEDUP-18).

Base branch

This PR targets refactor/vulkan-template-add-variant (#272), not
master — it depends on the _add_variant() helper. Will retarget
master once #272 lands.

Test plan

• ninja -C build-vulkan clean
• Netflix-pair smoke: integer_vif_scale0..3 means 0.364, 0.767, 0.863, 0.916; integer_vif mean 0.446 across 48 frames
• pre-commit run --files <touched> clean

Six deep-dive deliverables (ADR-0108)

• (1) Research digest: no digest needed: routine refactor
• (2) Decision matrix: no alternatives: only-one-way fix
• (3) AGENTS.md invariant note: see docs/rebase-notes.md entry 0119 — variants destroyed before bundle (same rule as ssim/psnr_hvs)
• (4) Reproducer / smoke-test command: see Test plan above
• (5) CHANGELOG fragment: entry under ### Changed
• (6) Rebase note: entry 0119

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR refactors vif_vulkan to use the shared Vulkan kernel-template pipeline abstraction and _add_variant() helper, reducing duplicated pipeline-management state while preserving the existing per-scale specialization behavior.

Changes:

• Replaced manually managed Vulkan pipeline/layout/shader/descriptor-pool state in vif_vulkan.c with VmafVulkanKernelPipeline plus three sibling scale variants.
• Added a vif_scale_pipeline() accessor and updated descriptor set / pipeline binding sites to use the new template-owned resources.
• Documented the migration and destruction-order invariant in docs/rebase-notes.md and CHANGELOG.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
libvmaf/src/feature/vulkan/vif_vulkan.c	Migrates VIF Vulkan pipeline creation, variant handling, binding, and teardown to the shared kernel-template abstraction.
docs/rebase-notes.md	Records the rebase/migration details and the destroy-variants-before-bundle invariant.
CHANGELOG.md	Adds a changelog entry describing the Vulkan refactor and expected unchanged numerical behavior.

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