fix: close relay loop from prefixed dedup mismatch

The dispatcher deduped on inbound text but relayed a prefixed copy
("[net/sender] text"), so a re-heard relayed message hashed differently
and was re-relayed, stacking another prefix each hop — an unbounded loop,
exactly what the dedup was meant to prevent.

Add MessageBus.mark_seen() and record the on-air relayed string before
sending, so the bridge recognises and drops its own relayed copies.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

host-bridge/main.py

@@ -82,11 +82,16 @@ def main() -> int:
             if bus.is_duplicate(message.text):
                 continue
             print(f"[{message.network}] {message.sender}: {message.text}")
+            relayed = f"[{message.network}/{message.sender}] {message.text}"
+            # Record the exact on-air string so that if this relayed copy is
+            # re-heard from another network it's recognised as a duplicate and
+            # not re-relayed (which would stack another prefix each hop).
+            bus.mark_seen(relayed)
             for network, adapter in adapters.items():
                 if network == message.network:
                     continue  # don't echo back onto the network it came from
                 try:
-                    adapter.send(f"[{message.network}/{message.sender}] {message.text}")
+                    adapter.send(relayed)
                 except Exception as exc:  # noqa: BLE001
                     print(f"  ! failed to relay onto {network}: {exc}", file=sys.stderr)
     except KeyboardInterrupt: