Make cutout coverage phase deterministic across backends

[proggeramlug]

Sponza and skinned/alpha motion failed the approved portable images on Windows because Metal and Vulkan produced different Bayer coverage grids from floating-point exp2 followed by floor. Exact UV and cutout-input captures locate the difference before TAA; the observed Metal extent predicts every inspected threshold away from integer LOD boundaries.

Derive the scene and shadow phase extents from the final source texel index with integer arithmetic. This preserves the approved Metal grid across backends, handles odd texture dimensions and coarse LODs, and leaves alpha sampling and approved images unchanged.

Both focused Windows images now pass on Radeon 760M/Vulkan with matching modern/software-GI settings:

Scene	Previous SSIM	Corrected SSIM
Sponza	0.969274342	0.986160457
Skinned/alpha motion	0.931530774	0.990073442

Validation on 662a44f:

• The GPU regression executes twelve reference points in both production shader paths. Vulkan, DX12, and hosted Metal pass all 24 values; the old Vulkan shader fails the first case.
• Local contracts, strict lint, and the complete shared suite pass, including 91 reported golden-test passes and four ignored tests. Optional external-input fixtures remain separate requirements.
• All nine Radeon image gates pass in both complete runs. Reproducibility passes with 257/257 artifacts byte-identical. The runs retain overall FAIL status for postflight host-load rejections: Bistro in run 1; Bistro, draw/light stress, and weighted transparency in run 2. Their affected timing windows remain invalid.
• Both hosted Metal images pass. All 24 hosted checks pass; the two scheduled physical-hardware jobs are skipped. Named RTX 4080/Apple M1 Max timing qualification remains open.

Stacked on draft #157. Related: #128, #140, #153. Investigation and compatibility rationale.

Published report, raw captures, controls, and checksums.

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