[DRAFT] [DirectX] FeedbackTexture2DArray support

[alsepkow]

⚠️ DRAFT — not ready for pickup. Title still being refined; do not assign or start work yet.

Background

PR #1235 delivers FeedbackTexture2D (MinMip + MipRegionUsed; compute + graphics; WriteSamplerFeedback / WriteSamplerFeedbackLevel). The issue body of #1055 and its AC checklist both call out FeedbackTexture2DArray as a required variant — not delivered.

This was deliberately scoped out of #1235 because adding the array variant doubles the surface area (DX TEXTURE2DARRAY paired view, per-slice readback for the feedback aspect, additional test coverage), and #1235 is already large (+1389 LOC across descriptor heap, paired-resource binding, capability gate, readback path, 4 tests).

Acceptance criteria

• Add ResourceKind::FeedbackTexture2DArray to include/API/Enums.h; switch cases in the descriptor helpers (VK/MTL still llvm_unreachable until a feedback extension exists).
• DX: paired SRV/UAV creation with D3D12_SRV_DIMENSION_TEXTURE2DARRAY / D3D12_UAV_DIMENSION_TEXTURE2DARRAY on the paired resource; descriptor-heap binding matches.
• DX: extend the feedback readback path to iterate slices (mirrors the per-slice readback added in [DX] Unify multi-subresource texture upload + Texture2DArray multi-mip support (stacked) #1256 for regular Texture2DArray).
• Add at least one test per writer (WriteSamplerFeedback + WriteSamplerFeedbackLevel) covering MinMip + MipRegionUsed on a 2-slice array.

Notes

• Estimated 1–2 days.

Refs #1055, llvm/wg-hlsl#400.