Three buffer overruns: compound tree stacks, hull clip polygons, mesh cast stacks

[nicolas-maman]

I audited every fixed-size scratch block and buffer in the library against the real limits of what gets written into it. Three can be overrun.

1. Compound tree stack (most severe). A baked compound tree carried one traversal stack inside the compound block, 2 KB. Tree queries index a tree's stack per worker, at 12 KB per worker, so a query on any worker past the first wrote 12 KB × worker index beyond it, into the heap. The parallel continuous pass reaches this whenever bullets are swept against a compound.
   • Baked trees now point at the compound module's own per-worker stacks (query and distance), allocated with the module's scratch. The block no longer holds a stack.
   • dynamic_tree exports the per-worker sizes (query_stack_bytes, query_dist_bytes).
2. Hull clip polygons. They held 64 vertices, but an incident face of up to 64 sides clipped against a reference face of up to 64 can reach 128. Two 40-sided cylinders cap to cap at a twist give an 80-gon, and the last worker's buffer spilled past the allocation. They now hold 128, as triangle_manifold's already did.
3. Mesh casts. The ray and shape casts pushed the far child at every level with no guard, so a tree leaning right deeper than the 256-entry stack overran it. Every push in every mesh traversal is guarded now; a pathologically deep tree drops the far subtree instead of writing past the stack.

Tests, each run as a high worker index so an overrun lands past the allocation:

• test_compound_workers: compound ray cast as workers 1, 7, 31 and 63. Main crashes with an access violation.
• test_wide_faces: 40-sided cylinders collided as worker 63.
• test_deep_tree_cast: a ray cast through a 300-deep right spine as worker 63.

On main, the last two overrun into memory that happens to be writable, so they don't fail there; they guard the result going forward.

test_compound's permanent allocation count grows by the two new stacks. All 28 test programs pass locally on aether 0.772.0, with no warnings.