Gate touch_read on ISR flag instead of double-polling

touch_read() had two branches that both called getPoint() unconditionally
— the touch_data_ready flag served no purpose. Gate the I2C transaction
on the flag so the ISR finally pulls its weight: idle loops no longer
hit I2C, getPoint() runs only when the CST9220 actually raised the
TP_INT line.

Also scope touch_pressed/x/y to file-static (no cross-TU users) and
drop the stale 'permanent while we work on it' comment by the default
boot screen — splash-on-boot is documented in CLAUDE.md.

firmware/src/main.cpp

@@ -30,9 +30,9 @@ SensorQMI8658 imu;
 static UsageData usage = {};
 
 // ---- Touch interrupt + shared state ----
-volatile bool     touch_pressed = false;
+static volatile bool     touch_pressed = false;
-volatile uint16_t touch_x = 0;
+static volatile uint16_t touch_x = 0;
-volatile uint16_t touch_y = 0;
+static volatile uint16_t touch_y = 0;
 static volatile bool     touch_data_ready = false;
 
 static void IRAM_ATTR touch_isr(void) {
@@ -40,18 +40,7 @@ static void IRAM_ATTR touch_isr(void) {
 }
 
 static void touch_read() {
-    if (!touch_data_ready) {
+    if (!touch_data_ready) return;
-        int16_t tx[5], ty[5];
-        uint8_t n = touch.getPoint(tx, ty, touch.getSupportTouchPoint());
-        if (n > 0) {
-            touch_pressed = true;
-            touch_x = (uint16_t)tx[0];
-            touch_y = (uint16_t)ty[0];
-        } else {
-            touch_pressed = false;
-        }
-        return;
-    }
     touch_data_ready = false;
 
     int16_t tx[5], ty[5];
@@ -305,7 +294,6 @@ void setup() {
     // Show initial battery status
     ui_update_battery(power_battery_pct(), power_is_charging());
 
-    // Default to splash screen (permanent while we work on it)
     ui_show_screen(SCREEN_SPLASH);
 
     Serial.println("Dashboard ready, waiting for data on BLE...");