nx_bounds_perf.nx

nx_bounds_perf.nx -- the COST side of the spatial-safety rung. A hot loop over a fixed array [1024]i64 with a RUNTIME index, which is exactly the shape emit_bounds_check instruments. Compiled by two compilers that differ ONLY in the NX_BOUNDS_CHECK_LIVE const, the delta between the two runs IS the overhead -- no modelling, no estimate. Prints microseconds and the checksum (the checksum also keeps the loop alive against dead-code elimination, and proves both builds computed the SAME answer).