Visual regression: verify WebGL overlay paging on patternv0.46 past row 64

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Context

Follow-up from shader playback audit (PR #268). Circular hybrid shaders like patternv0.46.wgsl page rows with floor(playhead / numRows) * numRows. The WebGL overlay was fixed to fetch cell data from the current page instead of always rows 0..N-1.

This needs a visual regression check to confirm caps stay aligned with the WebGPU grid during playback and seek.

Test steps

1. Start dev/preview server
2. Select patternv0.46.wgsl (WebGPU + hybrid overlay)
3. Load a module with >64 rows (e.g. 4-mat_-_space_debris.mod)
4. Play past row 64, or seek to rows 64, 80, 96
5. Confirm frosted LED caps sit on the same cells as the WGSL grid (no offset/slide)

Automated option

npm run dev -- --host 0.0.0.0 --port 5173
npm run capture:trigger-tail   # or extend audit script for v0.46 paging

Capture screenshots at rows 0, 32, 64, 96 and compare overlay alignment.

Acceptance criteria

• Overlay caps track paged pattern data after row 64
• No horizontal drift between WebGPU chassis and WebGL caps
• Screenshots or capture script output attached to close this issue

Related

• PR Fix WebGL overlay placement and octave note-color parity #268
• scripts/audit-shader-modes.mjs

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JUL-800