fix: detect overlapping polygon and pill courtyards

[zkasuran]

Closes #185

What

checkCourtyardOverlap only collected pcb_courtyard_rect, pcb_courtyard_circle and pcb_courtyard_outline. pcb_courtyard_polygon and pcb_courtyard_pill were dropped by the filter, so two components whose polygon or pill courtyards overlap on the same layer passed the check with no error.

Why it matters

pcb_courtyard_polygon is emitted by real inputs. kicad-component-converter produces it when importing footprints and @tscircuit/eval carries it, so a KiCad footprint whose courtyard is a polygon was never checked for overlap. checkPcbComponentOverlap already treats pcb_courtyard_polygon as a courtyard, so the two overlap checks disagreed on what a courtyard is.

Change

• Add pcb_courtyard_polygon and pcb_courtyard_pill to the courtyard filter and the CourtyardElement type.
• getCourtyardPolygon returns the polygon points directly for pcb_courtyard_polygon and approximates a pill as a stadium polygon (two semicircular caps sampled the same way the circle courtyard is turned into a 32-gon).
• The different-layer skip is unchanged, so courtyards on opposite sides are still allowed to overlap.

Tests

New file tests/lib/check-courtyard-overlap/courtyard-polygon-pill-overlap.test.ts:

• overlapping polygon courtyards report one error, a far third component stays clear
• polygon courtyards on different layers stay clear
• overlapping pill courtyards report one error, separated pills stay clear
• an error-drawn SVG snapshot of the overlapping polygon courtyards

The three assertion tests fail before this change (0 errors) and pass after.

Verified locally before submitting

• bun test: 145 pass, 0 fail
• bunx tsc --noEmit: clean
• bunx biome format .: clean

AI assistance (Claude, Anthropic) was used in developing this change. The design, review and verification were done by the author. Verified locally before submitting: bun test (145 pass, 0 fail), bunx tsc --noEmit clean, biome format clean.