fix(search): reconcile directory watcher state

[aaaaaandrew]

Summary

• record shard identity only after a successful load so transient failures retry
• rescan after fsnotify registration to close the startup publication window
• reconcile after watcher errors, including queue overflow
• stop cleanly if fsnotify closes its channels

Test plan

• go test ./...
• go test -race ./search
• go vet ./search
• focused watcher tests repeated ten times