chat gets its own core connection, so a turn stops blocking every page (V-638)

ipc.Client serialises every call on one mutex, and mavweb routed 28
handlers plus /api/chat through the shared client. A turn is bounded
only by phraser.timeout, 60s in deploy, so a page load behind one could
wait that long.

/models already had its own connection for this reason. Chat gets the
third one. A failed dial logs and falls back to the shared client, which
is how it behaved before.

/api/ptt needs nothing here: it proxies to the voice port and never
touches this client.

A connection pool inside ipc.Client is the general form and stays
unbuilt until a second module is measured queueing.

cmd/mavweb/main.go

@@ -58,6 +58,12 @@ func main() {
 	// mutex, so sharing the connection would freeze every other page for the
 	// length of the load. See handleModels.
 	var swapConn modelController
+	// turnConn — a third connection, for POST /api/chat and nothing else, for
+	// the same reason /models has one (V-638). A chat turn routes, phrases and
+	// may act, bounded only by phraser.timeout at 60s, and every other handler
+	// on this server queues behind it on the shared client's one mutex. Nil ⇒
+	// chat shares the main connection, which is how it behaved before.
+	var turnConn ipc.CoreAPI
 	if *coreSock != "" {
 		c, err := ipc.DialWait(*coreSock, 60*time.Second)
 		if err != nil {
@@ -71,6 +77,12 @@ func main() {
 			defer sc.Close()
 			swapConn = sc
 		}
+		if tc, err := ipc.Dial(*coreSock); err != nil {
+			log.Printf("chat: third core connection failed (%v) — /api/chat will share the main one and a turn will block the other pages", err)
+		} else {
+			defer tc.Close()
+			turnConn = tc
+		}
 	}
 
 	// stepUpSession stays nil unless the passkey endpoints are wired below — it
@@ -208,7 +220,13 @@ func main() {
 	// decides how every utterance is routed and how every reply is worded.
 	mux.HandleFunc("/tools", gatedPage(handleTools))
 	mux.HandleFunc("/routines", gatedPage(handleRoutines))
-	mux.HandleFunc("/api/chat", gatedPage(handleChatAPI))
+	mux.HandleFunc("/api/chat", func(w http.ResponseWriter, r *http.Request) {
+		c := turnConn
+		if c == nil {
+			c = core
+		}
+		handleChatAPI(w, r, c, stepUpSession, *requireStepUp)
+	})
 	mux.HandleFunc("/api/revert", gatedPage(handleRevert))
 	mux.HandleFunc("/api/correct", gatedPage(handleCorrectAPI))
 	mux.HandleFunc("/models", func(w http.ResponseWriter, r *http.Request) {