Fix pulse shape and button responsiveness

- Pulse shape: instant-on at beat, quick fade to 0 by phase 0.25
  (was smooth sine wave - didn't show exact timing)
- Active LEDs no longer overridden by update() - they keep their
  set_led_state() color, restoring button press responsiveness
- Pixel 6 only pulses when not active (acts as visual metronome)
  When active as a loop trigger, stays at full brightness

src/led_stub.cpp

@@ -1,7 +1,6 @@
 #include "led_stub.h"
 #include "pixel_stomp_mux.h"
 #include <Arduino.h>
-#include <math.h>
 
 static PixelStompMux* mux_ptr = nullptr;
 
@@ -316,30 +315,20 @@ void DefaultLedStub::update() {
     last_pulse_update = now;
 
     float phase = (now % beat_interval_ms) / (float)beat_interval_ms;
-    float pulse = sinf(M_PI * phase);
 
-    bool active = false;
-    for (int i = 0; i < NUM_LEDS; i++) {
-        if (led_states[i].active) {
-            active = true;
-            float brightness = 0.5f + 0.5f * pulse;
-            uint32_t base = pad_base_colors[i];
-            uint8_t r = (uint8_t)(((base >> 16) & 0xFF) * brightness);
-            uint8_t g = (uint8_t)(((base >> 8) & 0xFF) * brightness);
-            uint8_t b = (uint8_t)((base & 0xFF) * brightness);
-            mux_ptr->set_led_color(i, r, g, b);
-        } else if (i == 6) {
-            active = true;
-            float brightness = 0.3f + 0.7f * pulse;
-            uint32_t base = pad_base_colors[6];
-            uint8_t r = (uint8_t)(((base >> 16) & 0xFF) * brightness);
-            uint8_t g = (uint8_t)(((base >> 8) & 0xFF) * brightness);
-            uint8_t b = (uint8_t)((base & 0xFF) * brightness);
-            mux_ptr->set_led_color(6, r, g, b);
-        }
-    }
+    // Instant-on at beat, quick fade to 0 by phase 0.25
+    float pulse = 1.0f - phase * 4.0f;
+    if (pulse < 0.0f) pulse = 0.0f;
 
-    if (active) {
+    // Active LEDs keep their last set_led_state() color — don't override.
+    // Pixel 6 pulses only when not active (visual metronome for timing).
+    if (!led_states[6].active) {
+        float brightness = 0.3f + 0.7f * pulse;
+        uint32_t base = pad_base_colors[6];
+        uint8_t r = (uint8_t)(((base >> 16) & 0xFF) * brightness);
+        uint8_t g = (uint8_t)(((base >> 8) & 0xFF) * brightness);
+        uint8_t b = (uint8_t)((base & 0xFF) * brightness);
+        mux_ptr->set_led_color(6, r, g, b);
         mux_ptr->show();
     }
 }