wire hive_discover: route prompts to workers discovered across the mesh #63

Discovery is half-built and unwired. hive_advertise fires on every peer hello, but hive_discover has zero production call sites — so humd can announce its bees to the mesh and then never hears about anyone else's.

Current state (verified on f968e39)

So explicit addressing works today (to: <hid> routes at humd/src/lib.rs:733, proven by sim/tests/eggs_on_the_hum.rs). What's missing is automatic discovery and selection.

Why hive_discover alone won't close this

1. Remote manifests have nowhere to live. manifests is keyed by local thrum client_id. A remote bee has no client_id — it exists only as a Hid. Needs a parallel HashMap<Hid, HiveManifest>.
2. Dispatch has to branch. Local hit → thrum_to(client_id). Remote hit → ensemble.route() with to: <hid>. One lookup, two transports.
3. The receiver can't answer a request. Inbound ensemble tones only forward replies by sid (:745). Worse, a prompt arriving with client_id == "ensemble" currently falls through to local session handling at :895 as though a local bee sent it. Needs an explicit decision: reject, or resolve Hid → local worker via the hid field already in HiveManifest.
4. Two selection mechanisms already exist — manifest-by-discovery and caps-by-pick_overflow_peer. Pick one source of truth instead of letting them drift.

Requirements

• remote: RwLock<HashMap<Hid, HiveManifest>> fed by hive_discover, keyed by hive name
• worker selection consults local manifests and remote, preferring local
• dispatch branches to thrum_to or ensemble.route on locality
• inbound remote prompt resolved to a local worker, or explicitly refused — never silently treated as local
• staleness: a hive that dies must leave remote (reuse the lease/evict_expired machinery from ensemble: bound every send, order the opening frame, prune dead routes #62), otherwise we dispatch into a black hole forever
• capability/free_slots preference where the manifest carries it

Acceptance

Two humds in sim: B's worker discovered over gossip by A, A prompts it without knowing B in advance, reply returns to A's session by sid. Follows the existing integration-test shape (eggs_on_the_hum.rs), runs in CI.

Local-only dispatch must be observably unchanged — no regression in the current 209-test suite.

Non-goals

DHT bootstrap for workers (Kademlia already routes peers), NAT traversal (iroh already handles it), any central registry.