fix: debounce WHO on JOIN to prevent flood on netsplit recovery

WHO doesn't support multiple targets (absent from TARGMAX on all
major IRCds). Replace per-nick WHO with a debounced per-channel WHO:
on JOIN, schedule WHO #channel after 2s delay. Subsequent JOINs
within the window reset the timer, so a netsplit producing dozens
of JOINs results in a single WHO.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/derp/bot.py

@@ -87,6 +87,7 @@ class Bot:
         self._admins: list[str] = config.get("bot", {}).get("admins", [])
         self._opers: set[str] = set()  # hostmasks of known IRC operators
         self._caps: set[str] = set()  # negotiated IRCv3 caps
+        self._who_pending: dict[str, asyncio.Task] = {}  # debounced WHO per channel
         self.state = StateStore()
         # Rate limiter: default 2 msg/sec, burst of 5
         rate_cfg = config.get("bot", {})
@@ -265,11 +266,16 @@ class Bot:
             if "*" in flags:
                 self._opers.add(f"{nick}!{user}@{host}")
 
-        # JOIN — WHO the joining user to detect oper status
+        # JOIN — debounced WHO to detect oper status without flooding
         if msg.command == "JOIN" and msg.nick and msg.nick != self.nick:
             channel = msg.params[0] if msg.params else ""
             if channel:
-                await self.conn.send(format_msg("WHO", msg.nick))
+                existing = self._who_pending.get(channel)
+                if existing and not existing.done():
+                    existing.cancel()
+                self._who_pending[channel] = self._spawn(
+                    self._delayed_who(channel), name=f"who:{channel}",
+                )
 
         # QUIT — remove departed nicks from oper set
         if msg.command == "QUIT" and msg.prefix:
@@ -298,6 +304,16 @@ class Bot:
         if msg.command == "PRIVMSG" and msg.text:
             self._dispatch_command(msg)
 
+    async def _delayed_who(self, channel: str, delay: float = 2.0) -> None:
+        """Send WHO after a delay, debouncing rapid JOINs (e.g. netsplit)."""
+        try:
+            await asyncio.sleep(delay)
+            await self.conn.send(format_msg("WHO", channel))
+        except asyncio.CancelledError:
+            pass
+        finally:
+            self._who_pending.pop(channel, None)
+
     async def _handle_ctcp(self, msg: Message) -> None:
         """Respond to CTCP VERSION, TIME, and PING queries."""
         text = msg.text.strip("\x01")