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/main.rs

@@ -1,3 +1,29 @@
+use std::process;
+
 fn main() {
-    println!("Hello, world!");
+    let path = match std::env::args().nth(1) {
+        Some(p) => p,
+        None => {
+            eprintln!("Usage: xex2tractor <file.xex>");
+            process::exit(1);
+        }
+    };
+
+    let data = match std::fs::read(&path) {
+        Ok(d) => d,
+        Err(e) => {
+            eprintln!("Error reading {path}: {e}");
+            process::exit(1);
+        }
+    };
+
+    let xex = match xex2tractor::parse(&data) {
+        Ok(x) => x,
+        Err(e) => {
+            eprintln!("Error parsing XEX2: {e}");
+            process::exit(1);
+        }
+    };
+
+    xex2tractor::display::display_header(&xex.header);
 }