Distribute master OTA images to slaves over ESP-NOW.

After UART OTA the master reads the staged partition in 4 KiB blocks (200 B ESP-NOW chunks), waits for per-slave block ACKs, and fixes the final partial block. Slaves reuse ota_uart; send pacing and logging improve reliability.

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

main/proto/esp_now_messages.proto

@@ -1,5 +1,7 @@
 syntax = "proto3";
 
+import "nanopb.proto";
+
 package alox;
 
 enum EspNowMessageType {
@@ -9,6 +11,10 @@ enum EspNowMessageType {
   ESPNOW_HEARTBEAT = 3;
   ESPNOW_SET_ACCEL_DEADZONE = 4;
   ESPNOW_UNICAST_TEST = 5;
+  ESPNOW_OTA_START = 6;
+  ESPNOW_OTA_PAYLOAD = 7;
+  ESPNOW_OTA_END = 8;
+  ESPNOW_OTA_STATUS = 9;
 }
 
 message EspNowUnicastTest {
@@ -33,6 +39,27 @@ message EspNowAccelDeadzone {
   uint32 client_id = 2; // 0 = all slaves; otherwise only matching slave_id applies
 }
 
+// Master → slave: begin OTA (erase inactive slot; slave replies ESPNOW_OTA_STATUS).
+message EspNowOtaStart {
+  uint32 total_size = 1;
+}
+
+// Master → slave: firmware chunk (up to 200 bytes); slave buffers 4 KiB before flash write.
+message EspNowOtaPayload {
+  uint32 seq = 1;
+  bytes data = 2 [(nanopb).max_size = 200];
+}
+
+// Master → slave: finalize OTA (flush, esp_ota_end, set boot partition).
+message EspNowOtaEnd {}
+
+// Slave → master: status / block ACK (same status codes as UART OtaStatusPayload).
+message EspNowOtaStatus {
+  uint32 status = 1;
+  uint32 bytes_written = 2;
+  uint32 error = 3;
+}
+
 message EspNowMessage {
   EspNowMessageType type = 1;
   oneof payload {
@@ -41,5 +68,9 @@ message EspNowMessage {
     EspNowSlavePresence heartbeat = 4;
     EspNowAccelDeadzone accel_deadzone = 5;
     EspNowUnicastTest unicast_test = 6;
+    EspNowOtaStart ota_start = 7;
+    EspNowOtaPayload ota_payload = 8;
+    EspNowOtaEnd ota_end = 9;
+    EspNowOtaStatus ota_status = 10;
   }
 }