Add -out option for file output and fix stdin detection logic

- Add -out/-o option to save decode/encode output to files
- Encoded files are saved as binary, console still shows hex strings
- Decoded XML files are saved as text
- Fix stdin detection logic to prioritize command line arguments
- Add missing System.Collections.Generic import for List<string> usage
- File extension auto-processing now supports -out option
- Clean up data folder with renamed files (data0-3.dump/xml)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Data/test_decode.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ns1:V2G_Message xmlns:ns1="urn:iso:15118:2:2013:MsgDef" xmlns:ns2="urn:iso:15118:2:2013:MsgHeader" xmlns:ns3="urn:iso:15118:2:2013:MsgBody" xmlns:ns4="urn:iso:15118:2:2013:MsgDataTypes">
+  <ns1:Header>
+    <ns2:SessionID>4142423030303831</ns2:SessionID>
+  </ns1:Header>
+  <ns1:Body>
+    <ns3:WeldingDetectionReq>
+      <ns3:DC_EVStatus>
+        <ns4:EVReady>True</ns4:EVReady>
+        <ns4:EVErrorCode>NO_ERROR</ns4:EVErrorCode>
+        <ns4:EVRESSSOC>100</ns4:EVRESSSOC>
+      </ns3:DC_EVStatus>
+    </ns3:WeldingDetectionReq>
+
+  </ns1:Body>
+</ns1:V2G_Message>