Add VFS performance optimizations: node cache, lookup cache, access handler

- Add inode→VfsNode* cache to skip splitPath + full tree walks on
  lookup/getattr/open (O(1) hash lookup instead of tree traversal)
- Combine canonicalChildName + snapshotForPath into single lookupChild
  function (one tree lock, one parent resolution, one child hash probe)
- Add userspace lookup cache keyed by (parent_ino, normalized_name) to
  absorb Wine's case-variant probes that bypass the kernel FUSE dcache
- Add mo2_access handler to avoid kernel fallback to lookup+getattr
- Invalidate all caches correctly on tree mutations (create/unlink/rename/mkdir)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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