Fix FXC layered-material compilation with explicit lookup LOD

[proggeramlug]

Layered-material pipelines fail to compile with DX12/FXC when a sheen lookup uses implicit derivatives inside varying light loops. FXC attempts to unroll the loops and reports X3570/X3511, preventing four existing rendering goldens from reaching their image assertions.

Sample the single-mip sheen lookup table at explicit level zero. The shared helper keeps its texture, sampler and coordinates; existing rendering assertions and baselines are unchanged. This is stacked on #164 and does not fix the separate WARP/DXIL concurrency defect.

Validation on the Radeon 760M:

• Complete DX12/FXC shared suite passes: 489 library tests, device negotiation, all 93 goldens and remaining integration checks.
• DX12/DXC and Vulkan each pass all 93 goldens, including the four previously failing material cases.
• Strict Clippy, formatting, repository contracts and diff checks pass.

The FXC raster goldens require the physical Radeon adapter. Ray-query goldens retain their explicit DXC requirement; some library helpers select software adapters. Existing ignored tests remain recorded. Hosted Tests run 34555112391 passes all 22 jobs, including actual Windows shared execution, the native build and all 20 example links. Hosted Windows goldens skip the CPU adapter; physical image evidence comes from the local Radeon and hosted Metal runs. Canonical Metal captures and quality contracts pass; named full-hardware quality jobs remain skipped. Native/browser starter startup, packaging, wider engine acceptance and unavailable hardware qualification remain open.

Relates to #140, #128 and #153. Local evidence includes the original compiler failures, the source-equivalence receipt between #163/#164, the exact candidate patch and complete validation logs.

Published report and evidence include original compiler failures, the exact tested shader patch and complete local/hosted results. ZIP SHA-256: 8fc1a6af07b975a14ecc4d3b09b35c3db1967038b590d8ebca869528a9810b7b.

[coderabbitai]

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