feat(wgpu): add event-driven actor emulation for WebGPU backend

Gap analysis item 3.2 Tier 1: WebGPU cannot do true persistent kernels
but can emulate actor semantics via rapid re-dispatch with persistent
GPU buffers.

- WgpuActorLoop: tick-based dispatch with state/input/output buffers
  - state_buffer persists across dispatches (actor state)
  - inject_message() / poll_responses() for host-GPU messaging
  - start()/stop()/tick()/run() lifecycle
- ActorLoopConfig: buffer sizes, tick interval, workgroup count
- ACTOR_LOOP_WGSL_TEMPLATE: echo actor demonstrating buffer contract
- Wire into WgpuKernel: activate() starts loop when mode=Persistent
- 8 tests (3 CPU + 5 GPU-ignored)

From the user's perspective, the API is identical to CUDA persistent
actors (send/recv/activate/terminate).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mivertowski