Vaillant VR_71: add keepValveOperable (4) to Mc{1,2,3}Operation status

The room controller writes status=4 in a short daily burst (~5 min) on
otherwise-off mixer circuits — a valve anti-seize / keep-valve-operable
maintenance run. The shared onoff type (0=off;1=on) doesn't cover it, so
HA's enum sensor rejected the value. Add a dedicated Values_McStatus enum
(off=0, on=1, keepValveOperable=4) for the three Mc status fields; pump
stays onoff. Verified: tsp compile --warn-as-error passes, emitted CSV =
0=off;1=on;4=keepValveOperable, tsp format clean.

Refs #620

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

Author: buliwyf42

src/vaillant/26.vr_71.tsp

@@ -67,7 +67,9 @@ namespace Vr_71 {
   @inherit(w_1)
   @ext(0x2, 0)
   model Mc1Operation {
-    status: onoff;
+    @values(Values_McStatus)
+    status: UCH;
+
     flowtempdesired: temp1;
 
     @in
@@ -80,7 +82,9 @@ namespace Vr_71 {
   @inherit(w_1)
   @ext(0x2, 1)
   model Mc2Operation {
-    status: onoff;
+    @values(Values_McStatus)
+    status: UCH;
+
     flowtempdesired: temp1;
 
     @in
@@ -93,7 +97,9 @@ namespace Vr_71 {
   @inherit(w_1)
   @ext(0x2, 0x2)
   model Mc3Operation {
-    status: onoff;
+    @values(Values_McStatus)
+    status: UCH;
+
     flowtempdesired: temp1;
 
     @in
@@ -137,6 +143,17 @@ namespace Vr_71 {
     on: 20,
   }
 
+  /**
+   * Mixer-circuit operation status. Value 4 is written by the controller in a
+   * short daily burst (~5 min) on otherwise-off circuits — a "keep valve
+   * operable" maintenance run (anti-seize). See issue #620.
+   */
+  enum Values_McStatus {
+    off: 0,
+    on: 1,
+    keepValveOperable: 4,
+  }
+
   /** included parts */
   union _includes {
     Errors_inc,