Isolate HybridLLM state on MainActor

[corasan]

Summary

• Split HybridLLM into a thin Nitro-facing wrapper and a @MainActor-isolated HybridLLMCore that owns the mutable LLM state.
• Bridge synchronous Nitro entry points through Task { @MainActor in ... } so getters, setters, stop, unload, stats/history access, and clearHistory use the same isolation boundary.
• Keep public Nitro method signatures unchanged while moving load, generate, stream, and streamWithEvents onto async core methods driven by Promise.async.
• Clean up Swift concurrency diagnostics by capturing generation parameters before container.perform closures and awaiting main-actor logging from tool execution tasks.

Testing

• bun --cwd package typecheck
• bun ios --device [device id]