Large models: per-instance detail and picking inside instanced cells #12

Above the 384 MiB merged-geometry cap, repeated shapes are drawn as instanced copies in spatially bounded cells (#8). Measured on an integrated 2-CU AMD GPU with 1 000 000 elements, compared with the earlier one-draw-per-instance renderer:

• orbit 95th percentile 20 fps (was 29.9);
• pick ~22 ms (was ~7 ms);
• still-frame CPU time 23 ms (was 4 ms).

Causes: detail is cut per cell from the cell's nearest point (so all instances of one shape in a cell share one size estimate), each InstancedMesh draw costs ~5 µs of CPU in three.js + ANGLE, and the cell extent is relative to the whole-model diagonal, so a single far georeferencing outlier makes cells unbounded again.

Ideas:

• cull and cut detail per instance inside a cell (CPU-side instance spheres, compacted instance list);
• bound cells by an absolute extent or by robust model statistics instead of the full diagonal;
• reduce per-draw CPU cost (fewer, larger units where it is safe).

Models below the cap (all merged; e.g. 200 000 elements / 19.2 M triangles) are not affected: 30 fps at full detail on the same GPU.