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>

README.md

@@ -2,12 +2,42 @@
 
 A tool for extracting and inspecting Xbox 360 XEX2 executable files, written in Rust.
 
+## Usage
+
+```sh
+xex2tractor <file.xex>
+```
+
+### Example Output
+
+```
+=== XEX2 Header ===
+Magic:            XEX2 (0x58455832)
+Module Flags:     0x00000001 [TITLE]
+Header Size:      0x00003000 (12288 bytes)
+Reserved:         0x00000000
+Security Offset:  0x00000090
+Header Count:     15
+```
+
 ## Building
 
 ```sh
 cargo build --release
 ```
 
+## Testing
+
+Place a sample XEX2 file at `tests/data/default.xex`, then run:
+
+```sh
+cargo test
+```
+
+## Documentation
+
+See [doc/xex2_format.md](doc/xex2_format.md) for the XEX2 file format specification.
+
 ## License
 
 This project is licensed under the MIT License. See [LICENSE](LICENSE) for details.