GPU: build the warp coverage mask per block with interval bounds

The coverage mask from 94fe300 evaluated the f64 warp for every output
pixel. coverage_mask now asks Warp::block_coverage for each 32 x 16 block
(the shared warp formulas evaluated with outward-rounded intervals, no
edge margin) and evaluates only undecided blocks near the edge per pixel
with Warp::covers; the bits are unchanged
(coverage_mask_matches_per_pixel_decision, incl. partial words and
padding bits; coverage_mask_with_the_horizon_in_view, a perspective
denominator exactly 0 inside the image, so the clamp and
undecided-division paths go through block_coverage and the mask).

Measured (release, synthetic source, embedded lens warp + CA +
perspective as in embedded_lens_perspective_edges_match):
6000x4000 ~30 ms -> 2.7 ms (32 threads, 11x), ~580 ms -> 24.5 ms
(1 thread, 24x); 2048x1365 3.6 ms -> 1.0 ms (3.5x), 67.5 ms -> ~5 ms
(12-14x). Rotate270 the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/gpu-pipeline.md

@@ -9,7 +9,17 @@ The CPU reference evaluates that boundary in double precision; GPU float
 rounding otherwise can turn a blank edge pixel into a photo pixel. The mask
 uses one bit per output pixel (rows padded to 32 bits). Sampling and color
 corrections remain on the GPU, and existing geometry stage caching reuses the
-result. This adds CPU coordinate evaluation only when warped geometry rebuilds.
+result. The mask is built per 32 × 16 block (`Warp::block_coverage`): the warp
+formulas are written once, generic over `lightcraft_geom::Real`, and evaluated
+with outward-rounded `Interval`s over the block. Those bounds enclose the f64
+result of every pixel in the block, so a block whose bounds lie inside the
+image edges (the same f64 comparisons, no margin), or beyond one of them, is
+filled at once; only blocks across the edge, or whose bounds are undecided (a
+perspective denominator that may vanish), evaluate each pixel (`Warp::covers`,
+the framing chain `Warp::frame` the CPU resample uses too). The bits equal the
+per-pixel decision. At 6000 × 4000 with lens warp + perspective the mask takes
+~2.7 ms (32 threads; ~24.5 ms on one) instead of ~30 ms (~580 ms), and only
+when warped geometry rebuilds.
 
 ## Backends, environment variables and troubleshooting (issue #136)
 wgpu loads the driver of **every** backend in an instance's set while it enumerates adapters — even