fix construction stuck-at-dirt: snap the shared clock on large drift + tab refocus (was a 0.02s/s crawl, useless after tab throttling)

fix: black-rectangle render bug — canvas size-sync (ResizeObserver + per-frame) + NaN-safe clamp + smooth FPV exit

NEMOCITY v0 — mock backend, gradio 6.16.0 (pre-SSR)