C++ binding implementation using ONNX Runtime

# C++ Binding for Magika

## Motivation

I needed fast, native file type detection for my terminal editor written in C++. While Magika has excellent Python and Rust implementations, there wasn't a C++ binding available. Rather than shell out to the CLI or embed Python, I built native C++ bindings using ONNX Runtime.

## What I've Built

A complete C++ implementation that:

- ✅ **Matches Python accuracy** - Uses the same ONNX model with identical feature extraction
- ✅ **High performance** - ~5ms inference, 1000+ files/sec throughput
- ✅ **Clean API** - PIMPL pattern, exception-safe, follows modern C++ practices
- ✅ **Full CLI** - Feature parity with Rust CLI (recursive, JSON output, colors, etc.)
- ✅ **Cross-platform** - Tested on Linux, macOS (planned: Windows)
- ✅ **Well documented** - Comprehensive README, API docs, and examples
- ✅ **Production ready** - Already integrated into my editor

## Implementation Highlights

**Architecture:**
- ONNX Runtime C++ API for inference
- Feature extraction ported from Python (lstrip/rstrip, padding, block_size)
- PIMPL pattern to hide ONNX dependencies from public headers
- CMake build system with proper install targets

**Key Files:**
```
cpp/
├── lib/ # Core library
│ ├── include/magika/ # Public headers
│ │ ├── magika.hpp
│ │ └── types.hpp
│ └── src/ # Implementation
│ └── magika.cpp
├── cli/ # Command-line tool
│ └── main.cpp
├── examples/ # Usage examples
└── README.md # Full documentation
```

**Testing:**
- Validated against Python implementation on `tests_data/`
- Matches detection results for all test files
- Handles edge cases (empty files, binary files, large files)

## Example Usage

**Library:**
```cpp
#include

magika::Magika detector("/path/to/models/standard_v3_3");
auto result = detector.identify_path("test.py");

std::cout << "Type: " << result.content_type << "\n" // "python"
<< "MIME: " << result.mime_type << "\n" // "text/x-python"
<< "Group: " << result.group << "\n" // "code"
<< "Confidence: " << result.score << "\n"; // 0.998
```

**CLI:**
```bash
$ build/cli/magika -r ~/projects --json
142 python
89 javascript
56 cpp
23 markdown
```

## Integration Example

Already working in my terminal editor:
```cpp
// Auto-detect file type on load
auto result = detector.identify_path(filename);
if (result.content_type == "python") {
enable_python_highlighting();
}
```

## Questions for Maintainers

Before submitting a full PR, I'd appreciate guidance on:

1. **Interest Level**: Would you accept a C++ binding as an official part of Magika?

2. **Dependency Management**: How should ONNX Runtime be handled?
- Current: User provides path via CMake (`-DONNXRUNTIME_DIR=...`)
- Alternative: FetchContent to auto-download
- Alternative: System package (apt-get, brew, vcpkg)

3. **Project Structure**:
- Should it be in `cpp/` directory (like `python/`, `js/`)?
- Or separate repo initially?

4. **CI/Testing**:
- Add GitHub Actions for C++ builds (Linux, macOS, Windows)?
- Integration tests comparing against Python output?
- What coverage is expected?

5. **API Design**: Any changes needed to match project conventions?
- Current: `Magika` class with `identify_path()`, `identify_bytes()`
- Exception-based error handling vs. Result types?

6. **Documentation**:
- Is the current README sufficient?
- Should API docs be generated (Doxygen)?

## Why This Adds Value

- **Performance-critical applications**: Game engines, embedded systems, high-throughput servers
- **Native integration**: Editors (Vim, Emacs plugins), file managers, backup tools
- **Ecosystem gap**: Go and JavaScript have bindings, C++ is a natural fit alongside Rust

## Preview

Working code available at: `https://github.com/moisnx/magika/tree/main/cpp`

## Next Steps

If you're interested, I can:
1. Clean up any remaining issues based on feedback
2. Add comprehensive CI workflows
3. Submit a formal PR with detailed changelog
4. Write integration guide for common C++ build systems

Looking forward to your thoughts! Happy to hop on a call to discuss if helpful.

Review the proposed implementation in the external cpp/ tree, starting with cpp/lib/include/magika/magika.hpp, cpp/lib/src/magika.cpp, cpp/cli/main.cpp, and cpp/README.md. Compare its stated tests against tests_data/ and determine whether the project wants this binding, including decisions on ONNX Runtime dependencies, project structure, CI, and API conventions. Done means a maintainer decision and an agreed integration scope.

Written by the indexing model from the issue text.