fix: KILL actually disconnects the victim; unify HTTP/IRC divergences

Addresses the 2026-08-10 FAIL review (findings 1-7, 9, 10).

KILL never terminated the victim's connection on either transport: both
paths called BroadcastQuit, which deletes the session row and tells the
victim's peers it quit, but leaves the victim holding a socket that looks
alive and silently delivers nothing while its nick is freed for reuse.
Service now owns a session-ID keyed registry of live wire connections that
ircserver populates at registration, and both KILL paths go through the new
Service.KillSession, which broadcasts the QUIT and then sends the victim a
KILL and ERROR :Closing Link before closing its socket. The victim's relay
goroutine is cancelled and its cleanup no longer re-broadcasts a QUIT for
an already-deleted session. TestIntegrationKill now asserts the victim
reads to EOF and is gone from NAMES and WHO, not just that an observer saw
the QUIT relay.

HTTP MODE <othernick> with no body answered with the requester's own modes,
because the target check sat inside the mode-change branch. The check is
hoisted above the query/change split, and both transports now compare nicks
with EqualFold since IRC nicks are case-insensitive.

Service.QueryUserMode returned "+" for a database failure, making an
unreadable mode indistinguishable from an unset one; it now returns an
error, and both callers surface it. db.GetUserhostInfo likewise treated
every scan error as "nick not found"; only sql.ErrNoRows is skipped now.

The four new unsynchronized c.nick reads this branch introduced are read
through currentNick() under c.mu, and c.closed is now guarded everywhere
because KILL writes it from another client's goroutine. Conn.send takes a
write mutex, as a connection is now written to by three goroutines.

server.Server.Run was left with no in-tree callers when its body was
inlined into the fx OnStart hook; it is deleted rather than left to drift.

INFO and VERSION had two implementations that had already diverged: the
version string is now Service.ServerVersion and the INFO body is
Service.InfoLines, used verbatim by both transports. The ctx parameters on
handleVersion/handleAdmin/handleInfo/handleTime existed only to be
discarded and are gone.

internal/ircserver/conn.go

@@ -46,6 +46,12 @@ type Conn struct {
 	serverSfx string
 	commands  map[string]cmdHandler
 
+	// writeMu serializes writes to conn.  A connection is
+	// written to by its own read loop, by its relay
+	// goroutine, and — when an operator KILLs it — by
+	// another client's goroutine.
+	writeMu sync.Mutex
+
 	mu        sync.Mutex
 	nick      string
 	username  string
@@ -62,6 +68,7 @@ type Conn struct {
 
 	lastQueueID int64
 	closed      bool
+	killed      bool
 	cancel      context.CancelFunc
 }
 
@@ -98,6 +105,47 @@ func newConn(
 	return conn
 }
 
+// Disconnect terminates the connection on behalf of an
+// operator KILL issued from either transport.  The victim
+// is told why, then its socket is closed so that the read
+// loop unblocks and serve() returns; without the close the
+// victim would keep a socket that looks alive but silently
+// delivers nothing.  Disconnect is called from the killer's
+// goroutine, never the victim's.
+func (c *Conn) Disconnect(reason string) {
+	c.mu.Lock()
+
+	if c.closed {
+		c.mu.Unlock()
+
+		return
+	}
+
+	c.closed = true
+	c.killed = true
+	nick := c.nick
+	host := c.hostname
+	c.mu.Unlock()
+
+	if nick == "" {
+		nick = "*"
+	}
+
+	c.sendFromServer(irc.CmdKill, nick, reason)
+	c.send(
+		"ERROR :Closing Link: " + host +
+			" (" + reason + ")",
+	)
+
+	// Stop the relay goroutine, which would otherwise keep
+	// polling a queue belonging to a deleted session.
+	if c.cancel != nil {
+		c.cancel()
+	}
+
+	c.conn.Close() //nolint:errcheck,gosec
+}
+
 // buildCommandMap returns a map from IRC command strings
 // to handler functions.
 func (c *Conn) buildCommandMap() map[string]cmdHandler {
@@ -131,10 +179,10 @@ func (c *Conn) buildCommandMap() map[string]cmdHandler {
 			c.handleCAP(msg)
 		},
 		"USERHOST":     c.handleUserhost,
-		irc.CmdVersion: func(ctx context.Context, _ *Message) { c.handleVersion(ctx) },
-		irc.CmdAdmin:   func(ctx context.Context, _ *Message) { c.handleAdmin(ctx) },
-		irc.CmdInfo:    func(ctx context.Context, _ *Message) { c.handleInfo(ctx) },
-		irc.CmdTime:    func(ctx context.Context, _ *Message) { c.handleTime(ctx) },
+		irc.CmdVersion: func(context.Context, *Message) { c.handleVersion() },
+		irc.CmdAdmin:   func(context.Context, *Message) { c.handleAdmin() },
+		irc.CmdInfo:    func(context.Context, *Message) { c.handleInfo() },
+		irc.CmdTime:    func(context.Context, *Message) { c.handleTime() },
 		irc.CmdKill:    c.handleKillCmd,
 		irc.CmdWallops: c.handleWallopsCmd,
 	}
@@ -185,7 +233,7 @@ func (c *Conn) serve(ctx context.Context) {
 
 		c.handleMessage(ctx, msg)
 
-		if c.closed {
+		if c.isClosed() {
 			return
 		}
 	}
@@ -194,22 +242,52 @@ func (c *Conn) serve(ctx context.Context) {
 func (c *Conn) cleanup(ctx context.Context) {
 	c.mu.Lock()
 	wasRegistered := c.registered
+	wasKilled := c.killed
 	sessID := c.sessionID
 	nick := c.nick
 	c.closed = true
 	c.mu.Unlock()
 
 	if wasRegistered && sessID > 0 {
-		c.svc.BroadcastQuit(
-			ctx, sessID, nick, "Connection closed",
-		)
+		c.svc.UnregisterWireConn(sessID, c)
+
+		// A KILLed session has already been broadcast and
+		// deleted by the killer; broadcasting again would
+		// fan out a QUIT for a session row that is gone.
+		if !wasKilled {
+			c.svc.BroadcastQuit(
+				ctx, sessID, nick, "Connection closed",
+			)
+		}
 	}
 
 	c.conn.Close() //nolint:errcheck,gosec
 }
 
+// isClosed reports whether the connection has been marked
+// for teardown, either by QUIT or by an operator KILL.
+func (c *Conn) isClosed() bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	return c.closed
+}
+
+// currentNick returns the connection's registered nick.
+// c.nick is written under c.mu during registration and
+// NICK changes, so every read must take the mutex.
+func (c *Conn) currentNick() string {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	return c.nick
+}
+
 // send writes a formatted IRC line to the connection.
 func (c *Conn) send(line string) {
+	c.writeMu.Lock()
+	defer c.writeMu.Unlock()
+
 	_ = c.conn.SetWriteDeadline(
 		time.Now().Add(writeTimeout),
 	)
@@ -392,7 +470,10 @@ func (c *Conn) completeRegistration(ctx context.Context) {
 			"failed to create session", "error", err,
 		)
 		c.send("ERROR :Internal server error")
+
+		c.mu.Lock()
 		c.closed = true
+		c.mu.Unlock()
 
 		return
 	}
@@ -403,6 +484,10 @@ func (c *Conn) completeRegistration(ctx context.Context) {
 	c.registered = true
 	c.mu.Unlock()
 
+	// Make this connection reachable by session ID so that
+	// KILL from either transport can disconnect it.
+	c.svc.RegisterWireConn(sessionID, c)
+
 	// If PASS was provided before registration, set the
 	// session password.
 	if c.passWord != "" && len(c.passWord) >= minPasswordLen {