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>

create_test4.c

@@ -0,0 +1,24 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main() {
+    const char* hex_string = "8098021050908C0C0C0E0C50D10032018600A01881AE0601860C806140C801030800006100001881980600";
+    FILE* file = fopen("test4.exi", "wb");
+    
+    if (!file) {
+        printf("Error creating test4.exi\n");
+        return -1;
+    }
+    
+    size_t len = strlen(hex_string);
+    for (size_t i = 0; i < len; i += 2) {
+        unsigned int byte;
+        sscanf(&hex_string[i], "%2x", &byte);
+        fputc(byte, file);
+    }
+    
+    fclose(file);
+    printf("test4.exi created successfully (%zu bytes)\n", len/2);
+    return 0;
+}