Near-instant unchanged builds

Once now returns almost immediately when nothing relevant has changed. A build that already matches the cache no longer revalidates the whole graph on every invocation. Instead, Once records what changed since the last successful build and reuses the previous result when the sources, declared outputs, environment, and tools it observed are all unchanged.

Input checking is also cheaper. Once validates unchanged files by their metadata rather than rereading their contents, so a repeat build no longer pays to rehash the entire dependency tree. When a cached result is reused, only the outputs that actually differ are restored. Together these reduce both the time and the memory an unchanged or lightly changed build needs.

Correctness is preserved conservatively: any change to a watched source or output, a relevant environment variable, or an observed tool on disk falls back to a full validation before the fast path is trusted again.