PERF.37 BimCraft: the first click builds the inverse index on the page thread (5 to 12 s), picking raycasts every triangle, a section plane rebuilds all caps

Measured in /use 0.8.1 on 22.5 / 115 / 201 MB IFC (tools/perf/apps3d-bimcraft.mjs): the first property lookup calls ifc/model.js:173 inverse() (api.js:347-421) over the whole file: one 5.1 s task at 115 MB, 7.7 s plus properties = 12.1 s blocked at 201 MB. hitAt (viewer3d.js:833) has no bounding-volume tree although three-mesh-bvh is vendored: 47 to 133 ms per raycast at 683k triangles, 180 to 508 ms at 2.26 M, on every hover. setSection rebuilds every batch's stencil caps (updateCaps viewer3d.js:504): 1.4 s, 3.9 s, 8.0 s. Change: build the inverse index in the scan worker (open-async.js) or in idle slices after open; computeBoundsTree per batch; cache per-batch cap geometry and cull batches by the plane. Open itself: blocking 9.0, 13.9, 18.5 s with longest tasks 3.0, 5.5, 8.2 s (StreamAllMeshes api.js:312 in one synchronous loop, appendGeometry ifc-model.js:238 into JS arrays, runEdges ifc-vis.js:111 at 1.1 s per million triangles, the property index put at open): that is PERF.8, now with 10x numbers. Healthy: batching (46 to 104 draw calls), render CPU 2 to 7 ms, the tree panel (19k nodes, setTree 12 ms), idle after open.