Migrate from Tokio Graceful Shutdown to Kameo actor framework

[RobbieMcKinstry]

This commit refactors all subsystems to use the Kameo 0.19 actor framework
instead of Tokio's graceful shutdown pattern. Key changes include:

• Update all actors to implement Kameo's Actor trait with proper Args and
  Error types
• Convert message handlers to use Kameo's Message trait with Context
• Replace custom mailbox handling with Kameo's unbounded mailbox
• Add proper actor linking for supervision hierarchies
• Implement graceful shutdown via stop_gracefully() and wait_for_shutdown()
• Remove deleted mail.rs and handle.rs files in favor of integrated
  actor message handling
• Add custom builders for complex actor initialization
• Fix doctest to mark private module example as ignored

All 44 unit tests pass with no warnings.