Add C/C++ FFI interface for native integration

[muk2]

Summary

This PR adds a stable C-compatible FFI layer that enables C/C++ applications to integrate feedtui as a library, addressing issue #12.

Features Added:

• Lifecycle APIs:

  • feedtui_init(config_path) - Initialize with config file
  • feedtui_init_with_config(toml_string) - Initialize with inline config
  • feedtui_run(handle) - Run the TUI (blocks until quit)
  • feedtui_shutdown(handle) - Clean shutdown and resource cleanup

• Utility Functions:

  • feedtui_get_last_error(handle) - Get detailed error messages
  • feedtui_version() - Get library version
  • feedtui_has_feature(name) - Check compile-time features

• Build Artifacts (with --features ffi):

  • libfeedtui.so / libfeedtui.dylib / feedtui.dll (shared)
  • libfeedtui.a / feedtui.lib (static)
  • include/feedtui.h (C header)

Documentation:

• Comprehensive guide: docs/FFI.md
• Build instructions for Linux, macOS, Windows
• CMake integration example

Examples:

• examples/cpp/main.cpp - Full C++ example with argument parsing
• examples/cpp/simple.c - Minimal C example
• examples/cpp/Makefile - Build system for examples

Key Design Decisions:

• Opaque handles prevent memory corruption across FFI boundary
• Panic-safe wrappers using catch_unwind
• Feature-gated with ffi flag (no overhead when not used)
• Thread-safety notes in documentation

Usage:

# Build library
cargo build --release --features ffi

# C++ example
cd examples/cpp
make
make run

Test plan

• Build library with cargo build --release --features ffi
• Verify shared library is created in target/release/
• Build C++ example with make in examples/cpp/
• Run example and verify TUI launches
• Press 'q' to quit and verify clean shutdown
• Test with custom config file
• Test error handling with invalid config

Closes #12

---

Generated with Claude Code

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.