Turtle erase mode cannot affect shapes

[needmorecowbell]

Problem

penmode("erase") in turtle scripts has no effect on shapes created via rectangle(), ellipse(), polygon(), or star(). Erase only works on line-segment strokes created by movement commands (forward, goto_pos, etc.).

This means scripts cannot erase through or delete shapes — the erase path simply passes over them with no effect.

Root Cause

Three architectural factors:

1. Separate storage — Shapes and strokes are stored in different CRDT collections (doc.addShape() vs doc.addStroke()). eraseAlongSegment() in TurtleDrawing.ts:132 only queries doc.getStrokes(), never doc.getShapes().

2. Incompatible geometry — lineIntersectsStroke() in turtleEraseUtils.ts performs segment-to-segment hit-testing against a stroke's point array. Shapes don't have point arrays — they have type/width/height/position, requiring different intersection logic.

3. Whole-object removal — The current erase model removes entire strokes that intersect the erase path. It does not split, clip, or carve. This means even if shapes were included, erasing "through" a rectangle would delete the entire rectangle, not cut a hole in it.

Relevant Files

• src/turtle/TurtleExecutor.ts — Lines 330-351 (erase dispatch), 358-406 (shape creation, ignores pen mode)
• src/turtle/TurtleDrawing.ts — Lines 132-168 (eraseAlongSegment, only checks strokes)
• src/turtle/turtleEraseUtils.ts — lineIntersectsStroke() (segment-based hit-testing)
• src/crdt/DrawfinityDoc.ts — addShape() / removeShape() (separate from stroke storage)

Proposed Fix (Two Tiers)

Tier 1: Erase deletes whole shapes (easy)

Add shape hit-testing to eraseAlongSegment():

• Query doc.getShapes() alongside doc.getStrokes()
• Implement bounding-box or geometric intersection tests for each shape type (rect → AABB test, ellipse → point-in-ellipse, polygon/star → bounding circle)
• Call doc.removeShape() for intersecting shapes
• Estimated scope: ~100-200 lines across 2-3 files

Tier 2: Pixel-level carving (hard)

Allow erasing to cut holes in shapes (e.g., carve a smiley face into a circle):

• Would require converting shapes to clipping paths or decomposing them into segments
• Fundamentally different rendering model — shapes would need to support boolean geometry operations
• Significant effort, likely a separate feature initiative

Workaround

Scripts can simulate carving by overdrawing shapes with the canvas background color instead of using erase mode. See the "Smiley Face" example in docs/turtle-scripts-pending.md.

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