Improve intersection performance

[nikku]

Proposed Changes

Speeds up path intersection computation without changing results (verified bit-identical output against main across 500 randomized path pairs):

• Precompute bezier segments + bounding boxes once per path instead of re-walking path2 and recomputing bounding boxes per segment pair
• Skip non-overlapping segment pairs via cheap bounding box precheck before expensive subdivision
• Compute arc rotation directly instead of per-call caching (cache was rebuilt on every call)
• Push results instead of repeated concat()

Measured on 60-segment mixed paths: ~25% faster overall, ~57% faster on arc-heavy paths.

Try out:

npm ci && npm run all

Checklist

Ensure you provide everything we need to review your contribution:

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  • Link to related issue(s), i.e. Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}
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[Copilot]

Copilot review overview

🔵 Needs a closer look

Consolidate the duplicate bounding-box intersection check in intersect.js.

Review effort: Lite
Findings: None

What changed in this PR

Improves path intersection performance while preserving output by reducing repeated geometry calculations.

Changes:

• Precomputes Bézier segments and bounding boxes.
• Adds bounding-box filtering and more efficient result collection.
• Simplifies arc rotation calculations and adds regression coverage.

File	Summary
test/​intersect.spec.js	Adds regression coverage for disjoint segment bounding boxes.
intersect.js	Implements intersection performance optimizations.

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