Improve ffi::specialize_for macro and add new ffi::specialize macro

[konard]

Summary

This PR implements improvements to the FFI specialization macros as requested in issue #11. The improvements focus on reducing verbosity and providing better developer experience while maintaining backward compatibility.

✨ Key Changes

1. Made convention parameter optional in ffi::specialize_for - defaults to C# convention
2. Added new ffi::specialize macro for embedded annotations with sensible defaults
3. Maintained full backward compatibility - existing code continues to work unchanged
4. Fixed compilation issues - removed deprecated box_syntax feature

🔄 Before and After

Before (Verbose):

#[ffi::specialize_for(
    types = "u8",
    types = "u16",
    types = "u32", 
    types = "u64",
    convention = "csharp",
    name = "doublets_constants_*"
)]

After (Option 1 - Simplified):

#[ffi::specialize_for(
    types = "u8",
    types = "u16",
    types = "u32",
    types = "u64", 
    name = "doublets_constants_*"
)]

After (Option 2 - Embedded Annotations):

#[ffi::specialize("doublets_constants_*")]

🧪 Test Plan

• Compiles successfully with modern Rust
• Backward compatibility verified - existing usage unchanged
• New macros generate correct FFI functions
• Examples provided demonstrating both approaches
• No breaking changes to existing codebase

📁 Files Changed

• doublets-ffi/ffi-attributes/src/lib.rs - Core implementation
• examples/ffi_test.rs - Working examples
• examples/usage_examples.md - Documentation

🎯 Implementation Details

Implemented both approaches suggested in the issue:

1. Option 1: Improved ffi::specialize_for with optional convention (implemented)
2. Option 2: New ffi::specialize macro for embedded annotations (implemented)

Both generate identical FFI functions with C# naming conventions for u8, u16, u32, u64 types.

Fixes #11

🤖 Generated with Claude Code