perf: optimize displacement export falloff — SoA layout, flat grid, merged passes

[Avatarsia]

Summary

• Integrate position lookup into existing dedup pass, eliminating a redundant iteration over all vertices
• Replace boundaryPositions array-of-arrays ([[x,y,z], ...]) with flat SoA layout (bpX, bpY, bpZ as Float64Array)
• Grid cells store boundary-point indices instead of [x,y,z] sub-arrays
• Replace bGrid = new Map() with bGrid = new Array(gridSize) for direct index access in the nearest-neighbor hot loop

Impact (100k triangles, ~5k boundary vertices): Eliminates ~5,000 mini-array allocations, replaces ~675,000 Map.get() calls with direct array indexing in the nearest-neighbor search, saves one full pass over 300k vertices.

Test plan

• node --check js/displacement.js passes
• Playwright: app loads without JS errors
• No console errors across all tests

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