fix: quench RGB after fan register writes

app/pigway_pi_control.py

@@ -194,7 +194,7 @@ def update_fan(t):
             pending_fan=(old,fan_level,float(t))
     fan_initialized=True
 
-def apply_pending_fan():
+def apply_pending_fan(keep_rgb_off=False):
     global pending_fan
     with pending_fan_lock:
         command=pending_fan
@@ -203,6 +203,10 @@ def apply_pending_fan():
     old,new,temp=command
     try:
         mcu(0x08,FAN_REG[new])
+        # Some revisions of the shared MCU briefly expose their previous RGB
+        # state after a fan-register write.  When the flash engine is already
+        # in its dark phase, immediately reassert OFF without changing phase.
+        if keep_rgb_off: mcu(0x07,0x00)
     except Exception:
         with pending_fan_lock:
             if pending_fan is None: pending_fan=command
@@ -782,7 +786,7 @@ class DisplayController:
                 # fan changes only while RGB is already dark so MCU-internal
                 # register handling cannot expose a transient unrelated color.
                 if not applied_flash or flash_on is False:
-                    with bus_lock: apply_pending_fan()
+                    with bus_lock: apply_pending_fan(keep_rgb_off=applied_flash)
                 if previous_error is not None: log("INFO","RGB_RECOVERED")
                 previous_error=None
             except Exception as e: