Add MAX98357A I2S speaker driver with tone generator

I2S audio output on GPIO 9/10/11 (BCLK/WCLK/DOUT). Sine wave tone
generator with predefined alert sounds: beep, alert, success, error.
Boot beep confirms speaker is working. Speaker shut down before deep
sleep to release I2S driver.

RNode_Firmware.ino

@@ -29,6 +29,9 @@
   SensorBHI260AP *bhi260 = NULL;
   bool bhi260_ready = false;
 
+  // MAX98357A I2S speaker
+  #include "Speaker.h"
+
   // CST9217 capacitive touch panel
   #include <touch/TouchDrvCST92xx.h>
   TouchDrvCST92xx touch;
@@ -304,6 +307,10 @@ void setup() {
         attachInterrupt(TP_INT, touch_isr, FALLING);
       }
 
+      // Init speaker (BLDO2 already enabled by PMU init)
+      speaker_init();
+      if (speaker_ready) speaker_beep();  // Boot audio feedback
+
       // BHI260AP init deferred — firmware upload takes ~10s at 1MHz I2C
       // and blocks serial communication during boot. Will be initialized
       // lazily from the main loop after radio is up.
@@ -2070,7 +2077,8 @@ void twatch_enter_deep_sleep(bool beacon_timer) {
     delay(150);  // Let the motor spin briefly before powering down
   }
 
-  // 1. Put display controller into sleep mode (must happen before SPI.end)
+  // 1. Shut down audio and display before closing buses
+  speaker_end();
   #if HAS_DISPLAY
     co5300_sleep();
   #endif

Speaker.h

@@ -0,0 +1,110 @@
+// MAX98357A I2S Speaker Driver — Minimal tone generator for T-Watch Ultra
+// Plays simple alert tones via I2S. No WAV files needed.
+
+#ifndef SPEAKER_H
+#define SPEAKER_H
+
+#if BOARD_MODEL == BOARD_TWATCH_ULT
+
+#include "driver/i2s.h"
+#include <math.h>
+
+#define SPK_BCLK   I2S_BCLK
+#define SPK_WCLK   I2S_WCLK
+#define SPK_DOUT   I2S_DOUT
+#define SPK_I2S_PORT I2S_NUM_0
+#define SPK_SAMPLE_RATE 16000
+#define SPK_TONE_BUF_SIZE 512
+
+static bool speaker_ready = false;
+
+bool speaker_init() {
+  i2s_config_t i2s_config = {};
+  i2s_config.mode = (i2s_mode_t)(I2S_MODE_MASTER | I2S_MODE_TX);
+  i2s_config.sample_rate = SPK_SAMPLE_RATE;
+  i2s_config.bits_per_sample = I2S_BITS_PER_SAMPLE_16BIT;
+  i2s_config.channel_format = I2S_CHANNEL_FMT_ALL_RIGHT;
+  i2s_config.communication_format = I2S_COMM_FORMAT_STAND_I2S;
+  i2s_config.dma_buf_count = 4;
+  i2s_config.dma_buf_len = 512;
+  i2s_config.use_apll = false;
+  i2s_config.tx_desc_auto_clear = true;
+
+  i2s_pin_config_t pin_config = {};
+  pin_config.bck_io_num = SPK_BCLK;
+  pin_config.ws_io_num = SPK_WCLK;
+  pin_config.data_out_num = SPK_DOUT;
+  pin_config.data_in_num = I2S_PIN_NO_CHANGE;
+
+  if (i2s_driver_install(SPK_I2S_PORT, &i2s_config, 0, NULL) != ESP_OK) {
+    return false;
+  }
+  if (i2s_set_pin(SPK_I2S_PORT, &pin_config) != ESP_OK) {
+    i2s_driver_uninstall(SPK_I2S_PORT);
+    return false;
+  }
+
+  speaker_ready = true;
+  return true;
+}
+
+void speaker_end() {
+  if (speaker_ready) {
+    i2s_driver_uninstall(SPK_I2S_PORT);
+    speaker_ready = false;
+  }
+}
+
+// Play a tone at given frequency (Hz) for given duration (ms) at volume (0-100)
+void speaker_tone(uint16_t freq, uint16_t duration_ms, uint8_t volume) {
+  if (!speaker_ready || freq == 0) return;
+
+  float vol = (float)volume / 100.0f;
+  int16_t amplitude = (int16_t)(16000 * vol);  // ~half of int16 max
+  uint32_t total_samples = (uint32_t)SPK_SAMPLE_RATE * duration_ms / 1000;
+  int16_t buf[SPK_TONE_BUF_SIZE];
+
+  uint32_t samples_written = 0;
+  while (samples_written < total_samples) {
+    uint32_t chunk = total_samples - samples_written;
+    if (chunk > SPK_TONE_BUF_SIZE) chunk = SPK_TONE_BUF_SIZE;
+
+    for (uint32_t i = 0; i < chunk; i++) {
+      float t = (float)(samples_written + i) / (float)SPK_SAMPLE_RATE;
+      buf[i] = (int16_t)(amplitude * sinf(2.0f * M_PI * freq * t));
+    }
+
+    size_t bytes_written = 0;
+    i2s_write(SPK_I2S_PORT, buf, chunk * sizeof(int16_t), &bytes_written, portMAX_DELAY);
+    samples_written += chunk;
+  }
+
+  // Brief silence to flush DMA buffer
+  memset(buf, 0, sizeof(buf));
+  size_t dummy;
+  i2s_write(SPK_I2S_PORT, buf, sizeof(buf), &dummy, portMAX_DELAY);
+}
+
+// Predefined alert tones
+void speaker_beep() {
+  speaker_tone(1000, 100, 50);  // Short 1kHz beep
+}
+
+void speaker_alert() {
+  speaker_tone(800, 200, 70);
+  speaker_tone(1200, 200, 70);
+}
+
+void speaker_success() {
+  speaker_tone(523, 100, 40);  // C5
+  speaker_tone(659, 100, 40);  // E5
+  speaker_tone(784, 150, 40);  // G5
+}
+
+void speaker_error() {
+  speaker_tone(300, 300, 60);
+  speaker_tone(200, 400, 60);
+}
+
+#endif // BOARD_MODEL == BOARD_TWATCH_ULT
+#endif // SPEAKER_H