Implement pthread-based actor runtime with mailbox queuing

[ib823]

Summary

Implement a complete actor runtime system using POSIX threads (pthread), replacing stub implementations with functional mailbox-based message passing. Actors now have proper state management, thread synchronization primitives, and a ring-buffer mailbox for message queuing.

Changes

• Add #include <pthread.h> to generated C code headers
• Define riina_actor_type_t struct to store actor type metadata (init state and handler)
• Define riina_actor_t struct with full runtime state: actor ID, state, handler, ring-buffer mailbox, pthread mutex, condition variable, and thread handle
• Implement riina_actor_decl() to register actor types in a global type registry and return type ID
• Implement riina_actor_spawn() to create actor instances with initialized pthread synchronization primitives
• Implement riina_actor_send() with mutex-protected ring-buffer mailbox enqueuing and condition variable signaling
• Implement riina_actor_recv() to return current actor state
• Add comprehensive unit tests covering pthread includes, struct definitions, spawn initialization, mailbox operations, and type registry

Type

• New feature

Testing

• Added 5 new unit tests for actor runtime functionality:
  • test_emit_actor_pthread_include — verifies pthread header inclusion
  • test_emit_actor_struct_definition — verifies actor struct and synchronization primitives
  • test_emit_actor_spawn — verifies mutex/condition initialization on spawn
  • test_emit_actor_send_mailbox — verifies mailbox ring-buffer and locking
  • test_emit_actor_recv_returns_state — verifies state retrieval
  • test_emit_actor_decl_stores_type — verifies type registry storage

Checklist

• No external dependencies added (pthread is POSIX standard)
• No unsafe code in Rust (C code generation only)
• New tests added for new functionality

https://claude.ai/code/session_01XLbQ6A6N4cKbjXDJ1iaDTU