Zigzag engine (6802 lines, 184 tests):
- Construction 17 normalisation: working through dimension 3+
- Import from homotopy-rs JSON: working (scalar, two_scalars, half_braid)
- Piece extraction via Embedding/restrict_diagram: working
- Type checking pipeline: working (Eckmann-Hilton half_braid passes)
- Essential identity detection: validated with full 2-diagram test
Bugs found and fixed:
- assemble_factorisations losing cospan legs during reassembly
- RewriteN::slice() using source offsets instead of target indices
- singular_preimage() not handling passthrough heights
- restrict_rewrite() not accounting for accumulated cone offsets
- Embedding::preimage() using regular_preimage for Singular case
Added vis-engine-spec.md: visualization engine specification
- 6-layer architecture from math primitives to scene graph
- SVG renderer for 2D, WebGL2 for 3D, custom hit testing
- Spring constraint integration point for semiotic rendering
- No external dependencies - game engine approach
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
