Stream slave accel via ESP-NOW with master snapshot cache.

Slaves push BMA456 samples at 16ms when enabled; the master caches per
client and exposes ACCEL_SNAPSHOT and ACCEL_STREAM over UART. goTool adds
dashboard stream controls, HTTP accel-stream routes, and an external
WebSocket API with per-connection receive/interval and slave stream commands.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/client_registry.h

@@ -21,6 +21,14 @@ typedef struct {
   uint32_t version;
   /** Accel deadzone in raw LSB per axis (master copy for ESP-NOW config). */
   uint32_t accel_deadzone;
+  /** Latest accel from slave ESP-NOW stream (master only). */
+  bool accel_valid;
+  int16_t accel_x;
+  int16_t accel_y;
+  int16_t accel_z;
+  uint32_t accel_updated_at;
+  /** Host-enabled ESP-NOW accel stream to master. */
+  bool accel_stream_enabled;
 } client_info_t;
 
 #define CLIENT_REGISTRY_DEFAULT_ACCEL_DEADZONE 100u
@@ -63,4 +71,13 @@ esp_err_t client_registry_get_accel_deadzone(uint32_t client_id,
 /** Push deadzone to all active registry entries; returns count updated. */
 size_t client_registry_set_accel_deadzone_all(uint32_t deadzone);
 
+/** Store latest accel sample from a slave (matched by sender MAC). */
+esp_err_t client_registry_update_accel(const uint8_t mac[CLIENT_MAC_LEN],
+                                       uint32_t slave_id, int16_t x, int16_t y,
+                                       int16_t z);
+
+esp_err_t client_registry_set_accel_stream(uint32_t client_id, bool enabled);
+esp_err_t client_registry_get_accel_stream(uint32_t client_id, bool *enabled_out);
+size_t client_registry_set_accel_stream_all(bool enabled);
+
 #endif