Add PolygonSplitter for symmetric edge clipping and polygon-level classification

[TheBeachLab]

Summary

Addresses #51 — enables symmetrical clipping along connected edges by replacing CDTTriangleSplitter with a new PolygonSplitter that:

• Groups CDT sub-triangles into polygon regions via flood-fill over non-constraint adjacency
• Classifies each polygon region with a single raycast instead of one per sub-triangle
• Shares canonical intersection edge instances between both geometry sides to ensure symmetric vertex positions

Changes

• New PolygonSplitter class (src/core/PolygonSplitter.js) — extends CDT triangulation with getPolygonRegions() flood-fill grouping
• Updated performSplitTriangleOperations (src/core/operations/operations.js) — iterates polygon regions for bulk classification, removes dead connectivity traversal code
• Canonical edge sharing (src/core/operations/operationsUtils.js) — both intersection maps reference the same Line3 instance per intersection edge
• Removed CDTTriangleSplitter — PolygonSplitter replaces it as the useCDTClipping = true backend

Test Plan

• All 36 existing + new unit tests pass (npm test)
• Volume correctness verified for SUBTRACTION, INTERSECTION, ADDITION with useCDTClipping = true
• Visual verification via simple example — no missing triangles or holes
• Performance improvement observed (~3x on simple SUBTRACTION case)
• Lint passes with 0 errors
• Build succeeds