fix(pgxl): fan mode needs the "setup" prefix — "setup fanmode=VALUE"

Confirmed live: "setup fanmode=CONTEST" is accepted (reply code 0) while the
bare "fanmode=CONTEST" we switched to earlier is rejected (0x50000015), so the
fan mode snapped back and never changed on the amp. Restored the "setup " verb
and now check the reply code, returning an error if the amp rejects the set.

internal/powergenius/powergenius.go

@@ -125,14 +125,19 @@ func (c *Client) SetFanMode(mode string) error {
 	default:
 		return fmt.Errorf("powergenius: invalid fan mode %q", mode)
 	}
-	// Set with the bare "key=value" verb the amp uses for its own status fields
-	// (same convention as "operate=1"). The earlier "setup fanmode=…" carried a
-	// bogus prefix the amp silently ignored, so the fan never changed and the next
-	// status kept reporting the old mode — the revert-to-Contest the operator saw.
-	reply, err := c.command("fanmode=" + m)
+	// The verb the amp wants is "setup fanmode=VALUE" — confirmed LIVE: with the
+	// "setup " prefix the amp replies code 0 (accepted), while the bare
+	// "fanmode=VALUE" we switched to earlier is rejected (reply code 0x50000015).
+	// That regression is what stopped the fan from changing; restoring "setup "
+	// fixes it.
+	reply, err := c.command("setup fanmode=" + m)
 	if err != nil {
 		return err
 	}
+	if code := replyCode(reply); code != "" && code != "0" {
+		applog.Printf("pgxl: set fanmode=%s REJECTED, reply=%q", m, reply)
+		return fmt.Errorf("powergenius: amp rejected fanmode=%s (code %s)", m, code)
+	}
 	applog.Printf("pgxl: set fanmode=%s reply=%q", m, reply)
 	c.statusMu.Lock()
 	c.status.FanMode = m // optimistic
@@ -141,6 +146,19 @@ func (c *Client) SetFanMode(mode string) error {
 	return nil
 }
 
+// replyCode returns the result code from an "R<id>|<code>|…" reply — "0" means
+// the amp accepted the command. Returns "" when the line isn't an R reply.
+func replyCode(reply string) string {
+	if !strings.HasPrefix(reply, "R") {
+		return ""
+	}
+	p := strings.SplitN(reply, "|", 3)
+	if len(p) < 2 {
+		return ""
+	}
+	return strings.TrimSpace(p[1])
+}
+
 // SetOperate puts the amp in OPERATE (1) or STANDBY (0).
 func (c *Client) SetOperate(on bool) error {
 	v := "0"