PERF.31 CivilCraft: Surface.outline() takes 20 to 30 s per call at 1 M triangles and runs again on every zoom bucket (63 % of a 30 s open)

Measured on the 500k-point survey (survey-500k.civilcraft.json and landxml-tin-500k.xml): tin.js:633-648 builds string keys for 3 M directed edges with no memo, and planScene calls outline() on every mmBucket change, so it is 63 % of the 30 s open, 79 % of the scene phase, and wheel frames have a p95 of 13 to 36 s on both files. Change: memoize the outline per surface version and use integer or half-edge keys (about 100 ms once). The biggest single win of the 2D track. Healthy around it: elevationAt 0.25 ms, profiles 4 to 10 ms, Delaunator about 0.3 s at 500k, indexed picking (16 ms sweeps).