ipc: promote startup socket-wait to a shared DialWait; use in all modules

The cold-start crash-loop wasn't mavweb-specific — mavpoll and mavcaldav also
ipc.Dial + exit on failure, so they crash-looped until core booted too. Moved
the retry into ipc.DialWait (capped backoff, bounded) and switched mavweb,
mavpoll, mavcaldav to it. mavweb's local dialCoreWithRetry is gone.

Test: server appears after DialWait starts → it waits and connects.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/ipc/client.go

@@ -55,6 +55,30 @@ func (c *Client) Close() error {
 	return c.conn.Close()
 }
 
+// DialWait is Dial with patience: it retries with capped backoff until the
+// socket is reachable or timeout elapses. Core loads models on boot and may
+// come up after its modules (compose depends_on orders container start, not
+// socket readiness), so a module that Dial'd once would crash-loop on a cold
+// start. Every core-dialing module should use this instead of Dial. Mid-life
+// core restarts are handled separately by the Client's own redial-on-drop.
+func DialWait(path string, timeout time.Duration) (*Client, error) {
+	deadline := time.Now().Add(timeout)
+	delay := 200 * time.Millisecond
+	for {
+		c, err := Dial(path)
+		if err == nil {
+			return c, nil
+		}
+		if time.Now().After(deadline) {
+			return nil, err
+		}
+		time.Sleep(delay)
+		if delay < 2*time.Second {
+			delay *= 2
+		}
+	}
+}
+
 // call — the single request/response engine. Serialized by c.mu so a frame
 // and its reply always pair up; no interleaving to disambiguate. A wire
 // RpcError is rehydrated into the matching package sentinel (errors.Is works