feat: add knockout rendering for silkscreen text

[buildingvibes]

Summary

• Add knockout rendering support for pcb_silkscreen_text elements in the 3D viewer
• When is_knockout is true, draws a filled silkscreen rectangle with the text cut out
• Supports knockout_padding for configurable padding around the text
• Handles rotation and bottom layer rendering
• Matches the same approach used for pcb_copper_text knockout (canvas destination-out compositing)

This completes the 3d-viewer checklist item from tscircuit/tscircuit#770.

Changes

src/utils/silkscreen-texture.ts

• Added knockout branch in the silkscreen text rendering loop
• When is_knockout is set, computes padding (with sensible defaults based on font size), draws a filled rectangle at the anchor position, then cuts out the text using globalCompositeOperation = "destination-out"
• Normal (non-knockout) text rendering is unchanged

stories/SilkscreenText/SilkscreenTextKnockout.stories.tsx

• New story demonstrating knockout silkscreen text with: default padding, custom padding, rotation, and bottom layer rendering side by side with normal text

Test plan

• Verify knockout story renders correctly in Storybook (filled rect with text cut out)
• Verify normal silkscreen text is unchanged
• Verify bottom layer knockout renders correctly
• Verify rotated knockout text renders correctly
• Verify custom padding works
• Build passes (bun run build)
• Format passes (bun run format:check)

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Project	Deployment	Actions	Updated (UTC)
3d-viewer	Ready	Preview, Comment	Feb 10, 2026 8:09am

[buildingvibes]

Hi! Following up - all CI checks pass (format, test, type-check, Vercel deploy). This adds knockout rendering for silkscreen text in the 3D viewer, matching the existing copper text knockout pattern. Happy to adjust anything!