perf: O(1) edge-category lookup in filterEdges

`getEdgeCategory` linear-scanned every category's type array (and re-ran
`Object.entries(EDGE_CATEGORY_MAP)`) for every edge, inside `filterEdges`.
Build a reverse `edgeType → category` index once at module load; lookup is
now O(1). First-category-wins order preserved.

Verified [executed]: byte-identical to the scan over all known edge types
plus unknowns (0 mismatches).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>