fix: handle fatal rollup watch events

[landeqiming666]

Summary

• handle Rollup watch FATAL events the same way as watch ERROR events
• add a focused watch-mode regression test that mocks Rollup's watcher instead of starting a long-running watch process
• keep the existing ERROR behavior covered

This addresses #184. It is intentionally close to the older #229 approach, but adds test coverage without requiring a real watch process to stay alive during Jest.

Test Plan

• npx --yes yarn@1.22.22 test:unit --runInBand
• npx --yes yarn@1.22.22 types
• git diff --check

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #184: bili hangs on watch mode when Rollup emits a FATAL event

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