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/proto/esp_now_messages.proto

@@ -17,6 +17,8 @@ enum EspNowMessageType {
   ESPNOW_OTA_STATUS = 9;
   ESPNOW_FIND_ME = 10;
   ESPNOW_RESTART = 11;
+  ESPNOW_ACCEL_SAMPLE = 12;
+  ESPNOW_SET_ACCEL_STREAM = 13;
 }
 
 message EspNowUnicastTest {
@@ -52,6 +54,20 @@ message EspNowAccelDeadzone {
   uint32 client_id = 2; // 0 = all slaves; otherwise only matching slave_id applies
 }
 
+/** Master → slave: enable/disable periodic accel ESP-NOW stream (~16 ms). */
+message EspNowAccelStream {
+  bool enable = 1;
+  uint32 client_id = 2;
+}
+
+/** Slave → master: latest BMA456 sample (sent ~every 16 ms). */
+message EspNowAccelSample {
+  uint32 slave_id = 1;
+  sint32 x = 2;
+  sint32 y = 3;
+  sint32 z = 4;
+}
+
 // Master → slave: begin OTA (erase inactive slot; slave replies ESPNOW_OTA_STATUS).
 message EspNowOtaStart {
   uint32 total_size = 1;
@@ -87,5 +103,7 @@ message EspNowMessage {
     EspNowOtaStatus ota_status = 10;
     EspNowFindMe find_me = 11;
     EspNowRestart restart = 12;
+    EspNowAccelSample accel_sample = 13;
+    EspNowAccelStream accel_stream = 14;
   }
 }