Scope open-registration and rename/remove ordering to overlapping paths;
let descriptor mutations and directory creation proceed without that lock.
Prove blocked opens/writes cannot stall unrelated namespaces, including a
queued rename, and retain post-rename descriptor notifications. Document
handler-owned watch lifetimes and narrow paths; test natural idle exit.
Card: SAU-359
SAU-359 implementer
0c1ef74f6e