OctreeVolume: fail loudly on cell-pool exhaustion instead of hanging

OctreeVolume: fail loudly on cell-pool exhaustion instead of hanging

getNewCellPointer looped forever when the pool was full (wrap-rescan
with no exit). Two guards now: the usedCellsCount fast path (made honest
— the master cell at index 0 is now counted at initWorld) and an
authoritative full-scan-cycle check that throws IllegalStateException
with capacity + remediation in the message. Verified with a tiny-pool
harness: 7 allocations + master fill 8 cells, the 9th throws (was: hang).

Also: class javadoc now states the subsystem's demo-only status and the
traceCell coverage reality; OctreeVolumeTest (2 tests) pins the
exhaustion behavior with a timeout that fails on the old implementation.