Lower display brightness to probe peak-current freeze

Piotr observed the freeze correlates with digit '8' on the last
(units) tube position, regardless of which sensor is shown
(25.08 C, 48 %, 1018 hPa). '8' lights the most 14-seg segments of
any digit, and the units digit changes most often, so the last
position is both peak-current and the most-written-to. At brightness
15 (max) that peak current likely dips the supply / glitches the I2C
bus, which the in-ISR delay()/endTransmission() then turns into a
permanent hang.

This commit drops brightness 15 -> 4 as a cheap, reversible probe.
If freezes stop or get much rarer, the root cause is current/brownout
(fix: decoupling cap on display VCC, or separate supply, plus an I2C
bus-recovery/timeout) rather than purely the ISR timing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

button.ino

@@ -18,11 +18,11 @@ void buttonISRstate() {
     else if (countdown == 0) {
       // TODO: restart metronome timer to align countdown
       countdown = COUNTDOWN;
-      updateTube();
+      displayNeedsUpdate = true;
     } else {
       countdown = -1;
       digitalWrite(BUTTON_LED_PIN, LOW);
-      updateTube();
+      displayNeedsUpdate = true;
       // TODO: disable metronome timer
     }
   }

metronom.ino

@@ -37,18 +37,19 @@ volatile float sensorValue[SENSORS] = {};
 volatile uint displaySensor = TEMPERATURE;
 volatile bool displayNeedsUpdate = false;
 volatile bool tubeRotated = false;
+volatile bool sensorsDue = false;
 int tubeTimerID;
 
+/* I2C (BME280 reads, tube writes) must never run in interrupt context:
+   both libraries call delay(), which inside an ISR can spin forever
+   because SysTick (millis/micros) is blocked. ISRs only raise flags;
+   loop() does the actual bus work. */
 void TasksHandler(void) {
   TasksISRs.run();
+}
 
-  if (displayNeedsUpdate || tubeRotated)
-    updateTube();
-
-  if (tubeRotated) {
-    TasksISRs.changeInterval(tubeTimerID, SENSOR_TIMESHARE[displaySensor]);
-    tubeRotated = false;
-  }
+void scheduleSensors() {
+  sensorsDue = true;
 }
 
 void setup() {
@@ -62,7 +63,7 @@ void setup() {
 
   TaskTimer.attachInterruptInterval_MS(TASK_HANDLER_INTERVAL, TasksHandler);
   TasksISRs.setInterval(METRONOME_INTERVAL, runMetronome);
-  TasksISRs.setInterval(SENSOR_INTERVAL, readSensors);
+  TasksISRs.setInterval(SENSOR_INTERVAL, scheduleSensors);
   tubeTimerID = TasksISRs.setInterval(SENSOR_TIMESHARE[displaySensor], rotateTube);
 }
 
@@ -79,4 +80,17 @@ void runMetronome() {
   }
 }
 
-void loop() {}
+void loop() {
+  if (sensorsDue) {
+    sensorsDue = false;
+    readSensors();
+  }
+
+  if (displayNeedsUpdate || tubeRotated)
+    updateTube();
+
+  if (tubeRotated) {
+    TasksISRs.changeInterval(tubeTimerID, SENSOR_TIMESHARE[displaySensor]);
+    tubeRotated = false;
+  }
+}

tube.ino

@@ -6,7 +6,11 @@ char tubeText[5] = "";
 void initTube() {
   // Wire initialized by BME280 sensor
   tube.setTubeType(TYPE_4, TYPE_4_DEFAULT_I2C_ADDR);
-  tube.setBrightness(15);
+  // Brightness drives LED current. The freeze correlates with '8' on the last
+  // multiplexed digit (most segments lit -> peak current) at full brightness 15,
+  // which points at a supply/I2C-bus glitch. Lowered to probe that hypothesis;
+  // raise back toward 15 if the display is too dim and freezes don't return.
+  tube.setBrightness(4);
   tube.setBlinkRate(BLINK_OFF);
 }