vibe code the initial impl

main/audio_in.h

@@ -0,0 +1,19 @@
+#pragma once
+#include <stdbool.h>
+#include <stdint.h>
+#include "esp_err.h"
+#include "freertos/FreeRTOS.h"
+#include "feature_params.h"
+
+typedef struct {
+    const int16_t *pcm;   // FRAME_LEN samples, valid until audio_in_release_frame()
+    uint8_t idx;          // slot index to hand back
+    uint32_t seq;         // monotonically increasing frame number
+} audio_frame_t;
+
+esp_err_t audio_in_init(void);    // creates the I2S RX channel and the frame pool
+esp_err_t audio_in_start(void);   // starts the capture task (pinned to core 1)
+bool audio_in_get_frame(audio_frame_t *f, TickType_t wait);
+void audio_in_release_frame(uint8_t idx);
+uint32_t audio_in_frames_captured(void);
+uint32_t audio_in_frames_dropped(void);   // processing fell behind