feat: Add enhanced EXI viewer with XML decode capability

- Implement complete CurrentDemandReq/CurrentDemandRes parsing (24 total fields)
- Add enhanced_exi_viewer.c with detailed message analysis
- Support -decode option for clean XML output (file-ready format)
- Enable ISO1, ISO2, DIN codec support in build configuration
- Fix C99 compatibility issues in makefiles (change -ansi to -std=c99)
- Create test utilities for hex string to EXI conversion
- Generate test files: test3.exi (CurrentDemandRes), test4.exi (CurrentDemandReq)

Features:
* Dual output modes: detailed analysis (default) vs XML (-decode)
* Complete V2G message type detection and parsing
* Session ID display in hex and ASCII formats
* Voltage/current/power readings with proper units
* All optional fields and status flags supported

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

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

data/sessionSetupReq.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<v2gci_d:V2G_Message xmlns:v2gci_b="urn:iso:15118:2:2016:MsgBody"
+    xmlns:ns2="urn:iso:15118:2:2016:MsgDataTypes"
+    xmlns:v2gci_h="urn:iso:15118:2:2016:MsgHeader"
+    xmlns:xmlsig="http://www.w3.org/2000/09/xmldsig#"
+    xmlns:v2gci_d="urn:iso:15118:2:2016:MsgDef">
+    <v2gci_d:Header>
+        <v2gci_h:SessionID>0000000000000000</v2gci_h:SessionID>
+    </v2gci_d:Header>
+    <v2gci_d:Body>
+        <v2gci_b:SessionSetupReq>
+            <v2gci_b:EVCCID>01</v2gci_b:EVCCID>
+        </v2gci_b:SessionSetupReq>
+    </v2gci_d:Body>
+</v2gci_d:V2G_Message>

data/sessionSetupRes.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<v2gci_d:V2G_Message xmlns:v2gci_b="urn:iso:15118:2:2016:MsgBody"
+    xmlns:ns2="urn:iso:15118:2:2016:MsgDataTypes"
+    xmlns:v2gci_h="urn:iso:15118:2:2016:MsgHeader"
+    xmlns:xmlsig="http://www.w3.org/2000/09/xmldsig#"
+    xmlns:v2gci_d="urn:iso:15118:2:2016:MsgDef">
+    <v2gci_d:Header>
+        <v2gci_h:SessionID>0000000000000010</v2gci_h:SessionID>
+    </v2gci_d:Header>
+    <v2gci_d:Body>
+        <v2gci_b:SessionSetupRes>
+            <v2gci_b:ResponseCode>OK</v2gci_b:ResponseCode>
+            <v2gci_b:EVSEID>1234567</v2gci_b:EVSEID>
+            <v2gci_b:EVSETimeStamp>123456789</v2gci_b:EVSETimeStamp>
+        </v2gci_b:SessionSetupRes>
+    </v2gci_d:Body>
+</v2gci_d:V2G_Message>