Performance: move parsing to Web Worker for large bursts

[takker99]

Priority: P3
Category: Performance

Background:
Parsing currently runs on main thread; heavy bursts (thousands of frames) could block UI if extended. A Web Worker can offload CRC + frame assembly.

Goal:
Introduce optional worker-based parser path toggled automatically when buffered bytes exceed threshold.

Acceptance Criteria:

• Worker file implementing parse loop, posting parsed frames + errors back
• Main thread fallback if worker unsupported
• Benchmark: 10k small frames processed with <50% main thread blocking vs baseline
• Tests (integration) simulate worker messages (mock or real) ensuring identical outputs

Tasks:

1. Prototype worker (TypeScript) build via Vite
2. Define message protocol (input bytes chunk, flush, result)
3. Refactor ModbusClient to optionally delegate
4. Benchmark script (manual) results documented in issue comments
5. Minimal test verifying delegation logic

Definition of Done:
Worker path functional, tests green, documented trade-offs.