Add GPU-native actor system for 3D wave simulation

[mivertowski]

Implements a cell-as-actor paradigm where each spatial cell is an
independent actor that communicates with neighbors via message passing
(halo exchange) instead of shared memory access.

Key features:

• HaloMessage struct for neighbor-to-neighbor pressure exchange
• CellActorState for per-cell actor state management
• ActorTileControl for managing actor lifecycle and HLC timestamps
• K2K routing tables for 3D grid topology (6 neighbors per cell)
• Persistent CUDA kernel implementing actor message loop
• HLC (Hybrid Logical Clocks) for temporal alignment across cells
• ActorGpuBackend3D for host-side orchestration

The actor model is exposed as ComputationMethod::Actor which can be
selected via SimulationConfig:

let config = SimulationConfig::default()
.with_computation_method(ComputationMethod::Actor);

This demonstrates the ringkernel persistent kernel and K2K messaging
abstractions in a real-world physics simulation context.