Add reliable Android BLE ride recorder

main/trikke_ble_protocol.h

@@ -10,6 +10,7 @@ extern "C" {
 
 #define TRIKKE_BLE_FRAGMENT_HEADER_SIZE 8
 #define TRIKKE_BLE_ACK_SIZE             8
+#define TRIKKE_BLE_BEGIN_SESSION_SIZE   12
 
 // BLE data notifications carry a little-endian packet sequence, byte offset,
 // total TRK1 frame size, then the frame bytes at that offset. The unchanged
@@ -29,6 +30,20 @@ bool trikke_ble_decode_ack(
     size_t ack_size,
     uint32_t *packet_sequence);
 
+// A session begin is ASCII "BGN1" followed by a receiver-generated uint64
+// token. Repeating the same token is idempotent across BLE reconnects.
+bool trikke_ble_decode_begin_session(
+    const uint8_t *command,
+    size_t command_size,
+    uint64_t *session_token);
+
+// Notification delivery is active only after both the CCCD and session
+// authorization are ready. Returns true when the effective state changes.
+bool trikke_ble_update_subscription(
+    bool notify_enabled,
+    bool session_connection_ready,
+    bool *subscribed);
+
 #ifdef __cplusplus
 }
 #endif