Add filesystem handler interface and proxy support

[seveibar]

Summary

• add a promise-based FilesystemHandler interface plus in-memory implementation and helpers
• update circuit execution paths to accept filesystem handlers, proxying them across the worker boundary via Comlink
• document the new filesystem option and cover it with a worker integration test

Testing

• bunx tsc --noEmit
• bun test tests/features/filesystem-handler.test.ts

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Codex Task

[github-actions]

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