Library API: implement MagicDatabase builder pattern

[unclesp1d3r]

Summary

Implement the builder pattern for MagicDatabase as specified in Core Flow 5 (Library API - Advanced Usage).

Context

The spec defines a fluent builder API:
```rust
let db = MagicDatabase::new()
.with_config(config)?
.load("/usr/share/file/magic/Magdir/")?;
```

Current state: Only MagicDatabase::load_from_file() and MagicDatabase::load_from_file_with_config() exist. No builder pattern.

Acceptance Criteria

• MagicDatabase::new() creates empty database
• .with_config(config) applies and validates configuration, returns Result
• .load(path) loads magic rules from file or directory
• Existing load_from_file() and load_from_file_with_config() remain as convenience methods
• Builder is documented with rustdoc examples
• Unit tests for builder API

Files to Modify

• src/lib.rs - Add builder methods to MagicDatabase

References

• Core Flows spec: Flow 5 (Library API - Advanced Usage)
• Epic Epic: libmagic-rs Phase 1 MVP #24

[traycerai]

Warning

Rate limit exceeded. Please try after 2512 seconds.