Add C/C++ FFI Interface to Enable Native Integration of feedtui

[muk2]

feedtui is currently designed as a standalone Rust terminal application built with ratatui. To broaden adoption and enable integration into native systems workflows, it would be valuable to expose a stable C-compatible FFI layer so that C++ users can:
• Build feedtui from source and invoke it programmatically
• Embed or launch the TUI from existing C++ applications
• Distribute feedtui as a shared/static library usable outside the Rust ecosystem

This would allow non-Rust users to leverage the TUI without needing to understand Rust internals.

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Proposed Solution

Introduce a C ABI boundary that exposes a minimal, stable interface for interacting with feedtui.

1. Rust-side FFI Layer
   • Add a dedicated ffi module/crate
   • Use #[no_mangle] extern "C" functions
   • Expose lifecycle-style APIs, for example:
   • feedtui_init(config)
   • feedtui_run()
   • feedtui_shutdown()

2. Build Artifacts
   • Support building:
   • libfeedtui.so / libfeedtui.dylib
   • Optional static library
   • Provide a cargo build --features ffi path

3. C/C++ Headers
   • Generate or maintain a C header (feedtui.h)
   • Ensure:
   • Opaque handles instead of Rust types
   • Explicit ownership and memory rules
   • No Rust panics cross the FFI boundary

4. Example Usage
   • Provide:
   • Minimal C++ example (main.cpp)
   • Build instructions using cmake or clang++
   • Demonstrate:
   • Initializing the app
   • Running the TUI
   • Clean shutdown

Open Questions
• Should the FFI be gated behind a feature flag?
• Do we want to guarantee ABI stability across minor versions?
• Should we support launching feedtui as a library and a CLI?