feat: parse and display XEX2 main header (M1)

Implement XEX2 main header parsing with module flag decoding.
Add error handling, big-endian read utilities, CLI entry point,
and comprehensive unit + integration tests against a sample file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/display.rs

@@ -0,0 +1,16 @@
+/// Pretty-print formatting for parsed XEX2 structures.
+use crate::header::Xex2Header;
+
+/// Prints the XEX2 main header in a human-readable format.
+pub fn display_header(header: &Xex2Header) {
+    println!("=== XEX2 Header ===");
+    println!("Magic:            XEX2 (0x{:08X})", header.magic);
+    println!("Module Flags:     {}", header.module_flags);
+    println!(
+        "Header Size:      0x{:08X} ({} bytes)",
+        header.header_size, header.header_size
+    );
+    println!("Reserved:         0x{:08X}", header.reserved);
+    println!("Security Offset:  0x{:08X}", header.security_offset);
+    println!("Header Count:     {}", header.header_count);
+}