feat(dfu): add connection and MTU preflight checks

lib/model/shifter_types.dart

@@ -24,6 +24,10 @@ const int universalShifterDfuOpcodeAbort = 0x03;
 
 const int universalShifterDfuFrameSizeBytes = 64;
 const int universalShifterDfuFramePayloadSizeBytes = 63;
+const int universalShifterAttWriteOverheadBytes = 3;
+const int universalShifterDfuMinimumMtu =
+    universalShifterDfuFrameSizeBytes + universalShifterAttWriteOverheadBytes;
+const int universalShifterDfuPreferredMtu = 128;
 
 const int universalShifterDfuFlagEncrypted = 0x01;
 const int universalShifterDfuFlagSigned = 0x02;
@@ -107,6 +111,61 @@ class DfuUpdateProgress {
       state == DfuUpdateState.failed;
 }
 
+enum DfuPreflightFailureReason {
+  deviceNotConnected,
+  wrongConnectedDevice,
+  mtuRequestFailed,
+  mtuTooLow,
+}
+
+class DfuPreflightResult {
+  const DfuPreflightResult._({
+    required this.requestedMtu,
+    required this.requiredMtu,
+    required this.negotiatedMtu,
+    required this.failureReason,
+    required this.message,
+  });
+
+  final int requestedMtu;
+  final int requiredMtu;
+  final int? negotiatedMtu;
+  final DfuPreflightFailureReason? failureReason;
+  final String? message;
+
+  bool get canStart => failureReason == null;
+
+  static DfuPreflightResult ready({
+    required int requestedMtu,
+    required int negotiatedMtu,
+    int requiredMtu = universalShifterDfuMinimumMtu,
+  }) {
+    return DfuPreflightResult._(
+      requestedMtu: requestedMtu,
+      requiredMtu: requiredMtu,
+      negotiatedMtu: negotiatedMtu,
+      failureReason: null,
+      message: null,
+    );
+  }
+
+  static DfuPreflightResult failed({
+    required int requestedMtu,
+    required DfuPreflightFailureReason failureReason,
+    required String message,
+    int requiredMtu = universalShifterDfuMinimumMtu,
+    int? negotiatedMtu,
+  }) {
+    return DfuPreflightResult._(
+      requestedMtu: requestedMtu,
+      requiredMtu: requiredMtu,
+      negotiatedMtu: negotiatedMtu,
+      failureReason: failureReason,
+      message: message,
+    );
+  }
+}
+
 class ShifterErrorInfo {
   const ShifterErrorInfo({
     required this.code,