feat(client): re-resolve the server hostnames at the DNS TTL

hbc, hbc_mini.py and hbc_mini.c resolved the server once at startup, so an
address change needed a client restart. They now re-resolve every TTL seconds
(smallest A/AAAA TTL, floor 30s, 300s when unknown) and re-point, add or drop
connections to follow the new addresses; a failed lookup keeps the current ones.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

hbd/client/main.py

@@ -25,6 +25,7 @@ from .. import __version__
 
 # Import plugin system
 from .plugin import PluginRegistry, PluginLoader, InfoPlugin, MonitorPlugin
+from . import dnsttl
 from .connectivity import Connectivity, set_shared
 
 # Constants
@@ -95,6 +96,14 @@ class AsyncConnection:
             self.transport.close()
             self.transport = None
             self.protocol = None
+
+    def set_target(self, addr: str, af: int):
+        """Re-point at a new server address; the socket is reopened on the next send if the family changed."""
+        if af != self.af:
+            self.close()
+            self.af = af
+        self.addr = addr
+        self.logger = logging.getLogger(f"hbc.conn.{addr}")
     
     async def sendto(self, msg: dict, msg_id: str = "HTB"):
         """Send a message to the server.
@@ -500,6 +509,48 @@ def stop():
             task.cancel()
 
 
+async def dns_refresher(connections: List[AsyncConnection], hosts: List[str], port: int, af_filter: int,
+                        name: str, interval: int, connectivity: Optional[Connectivity], next_id: int):
+    """Re-resolve the server hosts every TTL seconds and follow address changes.
+
+    A failed lookup leaves the connections untouched. Addresses that disappear are re-pointed at
+    new ones where possible; the rest are retired, and surplus new ones get a connection.
+    """
+    logger = logging.getLogger("hbc.dns")
+    loop = asyncio.get_running_loop()
+    delay = await loop.run_in_executor(None, _ttl_delay, hosts, af_filter)
+    while running:
+        try:
+            if shutdown_event:
+                await asyncio.wait_for(shutdown_event.wait(), timeout=delay)
+                break
+            await asyncio.sleep(delay)
+        except asyncio.TimeoutError:
+            pass
+        resolved = await loop.run_in_executor(None, dnsttl.resolve, hosts, port, af_filter)
+        if resolved is None:
+            delay = dnsttl.FAILED_RETRY
+            continue
+        new, gone = dnsttl.retarget(connections, resolved)
+        for conn in gone:
+            if len(connections) > 1:
+                logger.info(f"Dropping server address {conn.addr}")
+                conn._dead = True
+                conn.close()
+                connections.remove(conn)
+        for af, addr in new:
+            conn = AsyncConnection(next_id, addr, port, af, name)
+            next_id += 1
+            logger.info(f"Adding server address {addr}")
+            connections.append(conn)
+            active_tasks.append(asyncio.create_task(heartbeat_sender(conn, interval, connectivity)))
+        delay = await loop.run_in_executor(None, _ttl_delay, hosts, af_filter)
+
+
+def _ttl_delay(hosts: List[str], af_filter: int) -> int:
+    return dnsttl.refresh_delay([dnsttl.lookup_ttl(h, af_filter) for h in hosts])
+
+
 async def cleanup(connections: List[AsyncConnection]):
     """Cleanup connections on shutdown."""
     logger = logging.getLogger("hbc.cleanup")
@@ -640,6 +691,9 @@ async def async_main(args, config):
         task = asyncio.create_task(heartbeat_sender(conn, interval, connectivity))
         active_tasks.append(task)
     
+    active_tasks.append(asyncio.create_task(
+        dns_refresher(connections, hb_hosts, hb_port, af_filter, iam, interval, connectivity, conn_id)))
+
     # Plugin collector (uses all connections, but we'll use first one)
     if connections and registry.get_enabled():
         task = asyncio.create_task(plugin_collector(connections[0], registry))