perf: index Pen snapping with reference parity

[dompm]

Summary

• build cached Pen vertex and segment indexes from immutable piece geometry
• use sorted X/Y ranges for vertex and alignment snapping
• use cached vertices and exact-coordinate adjacency for Shift and equal-length queries
• add randomized parity coverage across 6, 24, and 96 vertices

Why

Steady Pen movement should not repeatedly flatten every piece or scan unrelated geometry.

Stack

Depends on #126 (codex/interaction-viewport).

Verification

• 25 interaction tests pass
• randomized snap checks pass at 6, 24, and 96 vertices
• production build succeeds
• no new TypeScript errors beyond the documented baseline

This is a draft pending performance review and browser regression testing.

[dompm]

The index removes steady-state curve flattening and the randomized vertex/alignment checks pass, but equal-length parity still has an uncovered ordering regression.

• [P1] Preserve project piece order in equal-length queries. The reference implementation appends qualifying piece segments in pieces order, but queryLengthSnap populates its Set in active-draft-point order. If an earlier draft point touches later project piece B and a later point touches earlier piece A, B is queried first. With A offering length 10, B length 20, and a current length of 15, the reference chooses 10 while the index chooses 20. Collect membership first, then iterate qualifying piece indexes in project order.
• The randomized parity suite does not call queryLengthSnap; it covers vertex, ordinary alignment, and Shift alignment only. Add fixed and randomized equal-length parity cases, including equal-distance ties and reversed active-point/project-piece ordering.

Cached linear Shift alignment is permitted by the plan, so its full vertex scan is not itself a blocker unless tracing shows it is material.

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View logs

[dompm]

The project-order fix is correct and the reversed active-point regression now passes.

• [P2] Complete the requested randomized equal-length parity coverage. The randomized loop still exercises only vertex, ordinary alignment, and Shift alignment. queryLengthSnap is compared with the reference in one fixed case only. Add randomized active drafts that touch multiple pieces in varying order, including ties, and compare queryLengthSnap with findLengthSnap across the 6/24/96-vertex fixtures before calling the full snap parity run complete.