Fix 74HC165 protocol: match RoxMux library polarity

Key difference from RoxMux (the library designed for this hardware):
- LD pin starts HIGH, pulses LOW->HIGH to latch (was inverted)
- Data pin is INPUT (not INPUT_PULLUP) - 10k pullups on MUX board
- bitWrite instead of manual bit shifting

src/pixel_stomp_mux.cpp

@@ -1,18 +1,20 @@
 #include "pixel_stomp_mux.h"
 #include <Arduino.h>
 
+#define ROXMUX_DELAY 1
+
 PixelStompMux::PixelStompMux(uint8_t dat, uint8_t ld, uint8_t clk, uint8_t di)
     : pin_dat(dat), pin_ld(ld), pin_clk(clk), pin_di(di),
       strip(nullptr), last_button_state(0) {
 }
 
 void PixelStompMux::begin() {
-    pinMode(pin_ld, OUTPUT);
     pinMode(pin_clk, OUTPUT);
-    pinMode(pin_di, INPUT_PULLUP);
+    pinMode(pin_ld, OUTPUT);
+    pinMode(pin_di, INPUT);
 
-    digitalWrite(pin_ld, LOW);
     digitalWrite(pin_clk, LOW);
+    digitalWrite(pin_ld, HIGH);
 
     strip = new Adafruit_NeoPixel(NUM_LEDS, pin_dat, NEO_GRB + NEO_KHZ800);
     strip->begin();
@@ -22,19 +24,25 @@ void PixelStompMux::begin() {
 
     Serial.printf("[MUX] Init DAT=%d LD=%d CLK=%d DI=%d\n",
                   pin_dat, pin_ld, pin_clk, pin_di);
-    Serial.printf("[MUX] 74HC165 x2 daisy-chain for %d buttons\n", NUM_BUTTONS);
+    Serial.printf("[MUX] 74HC165 x2 daisy-chain, WS2812C x%d LEDs\n", NUM_LEDS);
-    Serial.printf("[MUX] WS2812C for %d LEDs\n", NUM_LEDS);
-    Serial.printf("[MUX] DI pin reads: %d\n", digitalRead(pin_di));
 }
 
 uint8_t PixelStompMux::read_buttons() {
-    digitalWrite(pin_ld, HIGH);
-    delayMicroseconds(20);
     digitalWrite(pin_ld, LOW);
-    delayMicroseconds(20);
+    delayMicroseconds(ROXMUX_DELAY);
+    digitalWrite(pin_ld, HIGH);
 
-    last_button_state = shift_in_74hc165();
+    uint8_t data = 0;
-    return last_button_state;
+    for (int i = 7; i >= 0; i--) {
+        bitWrite(data, i, digitalRead(pin_di));
+        digitalWrite(pin_clk, HIGH);
+        delayMicroseconds(ROXMUX_DELAY);
+        digitalWrite(pin_clk, LOW);
+        delayMicroseconds(ROXMUX_DELAY);
+    }
+
+    last_button_state = data;
+    return data;
 }
 
 bool PixelStompMux::is_button_pressed(uint8_t index) {
@@ -78,36 +86,17 @@ void PixelStompMux::dump() {
     }
 }
 
-uint8_t PixelStompMux::shift_in_74hc165() {
-    uint8_t data = 0;
-
-    for (int i = NUM_BUTTONS - 1; i >= 0; i--) {
-        if (digitalRead(pin_di)) {
-            data |= (1 << i);
-        }
-        digitalWrite(pin_clk, HIGH);
-        delayMicroseconds(2);
-        digitalWrite(pin_clk, LOW);
-        delayMicroseconds(2);
-    }
-
-    return data;
-}
-
 void PixelStompMux::probe() {
     Serial.println("[MUX] === Hardware Probe ===");
-
     Serial.printf("  DI (GPIO %d) raw: %d\n", pin_di, digitalRead(pin_di));
 
-    Serial.println("  Testing LD pin...");
+    Serial.println("  Testing LD pin (RoxMux protocol)...");
     digitalWrite(pin_ld, LOW);
     delayMicroseconds(10);
     Serial.printf("  LD=LOW,  DI=%d\n", digitalRead(pin_di));
     digitalWrite(pin_ld, HIGH);
     delayMicroseconds(10);
     Serial.printf("  LD=HIGH, DI=%d\n", digitalRead(pin_di));
-    digitalWrite(pin_ld, LOW);
-    delayMicroseconds(10);
 
     Serial.println("  Toggling CLK 16 times, reading DI...");
     for (int i = 0; i < 16; i++) {