Assert every declared state's bucket on each push, before the memberships that name it (novox/hq ADR 0201)

The raise at start was the only place buckets were asserted, so a module
registered and assigned since had none until the control plane restarted —
found on the first module to declare state.

internal/broker/jetstream.go

@@ -86,6 +86,13 @@ func pinnedTo(path string) (*tls.Config, error) {
 	return PinnedToFingerprint(want), nil
 }
 
+// OnConn is the JetStream handle over a connection the caller already holds — the control plane's
+// link — for asserting what the bus holds without dialling a second time.
+func OnConn(conn *nats.Conn) *JetStream {
+	js, _ := conn.JetStream()
+	return &JetStream{conn: conn, js: js}
+}
+
 // DialPinned is Dial with the server's certificate pinned by a fingerprint the caller already holds
 // — a module or a build machine that was handed one beside its credential, and has no file.
 func DialPinned(url, fingerprint string, opts ...nats.Option) (*JetStream, error) {