feat(voice): add on-device Miao Zhang Gui wake word

Add an optional local wake response prompt configuration. The prompt asset is intentionally not included yet.

components/platform/include/platform.h

@@ -49,6 +49,12 @@ typedef struct {
     void *user_data;
 } platform_multimodal_downstream_playback_callbacks_t;
 
+typedef void (*platform_multimodal_audio_pcm_observer_t)(
+        const int16_t *pcm,
+        size_t samples,
+        uint32_t sample_rate_hz,
+        void *user_data);
+
 #define PLATFORM_LAN_HTTP_PATH_MAX_LEN          96
 #define PLATFORM_LAN_HTTP_HOSTNAME_MAX_LEN      32
 #define PLATFORM_LAN_HTTP_REQUEST_BODY_MAX_LEN  1024
@@ -321,6 +327,24 @@ esp_err_t platform_multimodal_audio_start_uplink(
 esp_err_t platform_multimodal_audio_stop_uplink(int timeout_ms);
 bool platform_multimodal_audio_is_uplink_running(void);
 bool platform_multimodal_audio_is_initialized(void);
+/**
+ * @brief Register the single mono PCM observer for the active capture path.
+ *
+ * The callback runs synchronously in the platform capture task, outside the
+ * registration lock, and must remain bounded and non-blocking.
+ */
+esp_err_t platform_multimodal_audio_register_pcm_observer(
+        platform_multimodal_audio_pcm_observer_t observer,
+        void *user_data);
+/**
+ * @brief Remove a matching PCM observer and wait for in-flight callbacks.
+ *
+ * Negative timeout is invalid. Zero performs a non-blocking drain check.
+ */
+esp_err_t platform_multimodal_audio_unregister_pcm_observer(
+        platform_multimodal_audio_pcm_observer_t observer,
+        void *user_data,
+        int timeout_ms);
 esp_err_t platform_multimodal_audio_play_downstream_opus(const uint8_t *data, size_t len);
 esp_err_t platform_multimodal_audio_begin_downstream_response(void);
 esp_err_t platform_multimodal_audio_finish_downstream_response(void);