rustbridge-ffi - C ABI exports and FFI buffer management

This crate provides the FFI boundary layer:

• [FfiBuffer] for passing data across FFI
• [PluginHandle] for managing plugin instances
• C ABI exported functions (plugin_init, plugin_call, etc.)

FFI Functions

The following functions are exported with C linkage:

• plugin_init - Initialize a plugin instance
• plugin_call - Make a synchronous request to the plugin
• plugin_free_buffer - Free a buffer returned by plugin_call
• plugin_shutdown - Shutdown a plugin instance
• plugin_set_log_level - Set the log level for a plugin
• plugin_call_async - Make an async request (future)
• plugin_cancel_async - Cancel an async request (future)