Fix cache invalidation in an incremental static-site builder (transitive partials, config, deletions, same-second edits)

# Task: fix the incremental builder's cache invalidation `minibuild.py` builds `site/out/*.html` from `site/src/*.md` and caches what it built in `site/.cache.json`. Users report stale pages: editing a partial (even one included indirectly) doesn't update pages, changing `config.json` doesn't either, quick successive edits within the same second are missed, deleted pages leave their HTML behind, and deleting or hand-editing an output file isn't repaired. Fix the caching in `minibuild.py` so that after **any** sequence of changes, `python3 minibuild.py site` leaves `out/` exactly equal to a clean full build (same files, same bytes, no extra files), while still being incremental: a page is rewritten only if its output would change or its output file is missing/modified. Pages unaffected by a change must not be rewritten at all (the grader checks output file timestamps). Note: the grader resets the timestamps of all output files to an old value before each build, so detect missing/modified outputs by their content (e.g. a hash), not by their timestamps. Only config keys a page actually uses (`site_title` is used by every page; `{{var KEY}}` by the pages containing it, directly or through partials) should cause it to rebuild. Keep the rendering output and the command line unchanged; `build()` should still return the list of rebuilt page names. Standard library only. No network.