The host watches the whole world folder recursively with the notify crate
(any writer counts, .sausage-tmp-* skipped, ~100 ms coalescing per path).
A new or changed actor/<id>/actor.wasm (or new folder actor/<id>) is
booted as at start once quiet: new = spawn (tested first), changed =
upgrade. The broker `scan` request is gone; the boot pass stays, plus a
full pass when the OS watcher overflows.
New broker requests watch/unwatch {path} (saved in actor.json `watches`)
deliver fs_changed {path, op: write|create|remove|rename, to?} from `fs`,
not routed, dropped when the mailbox is full; overflow sends every watcher
one fs_changed {path:"/actor", op:"rescan"}.
Card #91 (multi-host step 1).
agent-pool-9