vfs: persistent scan cache to skip mod walk on warm boot

Saves the merged VfsTree to ~/.local/share/fluorine/vfs_cache/ keyed by
data_dir + mods + overwrite. On a hit, mounting reads the tree from a
single binary file instead of re-walking every mod, which dominates
mount time on heavy modlists.

Validation: modlist.txt mtime+size, data/overwrite dir mtime, mod-dir
mtimes in priority order. Any drift invalidates. Session-scoped state
(extra-file injection, plugin timestamp stamping) is re-applied after
load and never cached.

rebuild() also refreshes the cache so the next cold mount hits.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions