#33545 base red: the no-flicker test type-checks (unused Box, shadowed Event; from #33526)

Summary

Tip web-lint red since b695980afce2 (STATUS 8746), from #33526 (webviews/test/agent-pane-streaming-no-flicker.test.ts):

• eslint(no-unused-vars): the outer type Box is never used. Each page.evaluate body declares its own Box (code there runs in the page and cannot see outer types). This PR removes the outer one.
• 7 TypeScript errors (TS2339/TS2352/TS2769) in the page listener: the file declares its own type Event (the acpmux event, line 74), which shadows the DOM Event in onAnimation. The parameter is now globalThis.Event.

No test logic changes.

Verification

• bunx vp lint test/agent-pane-streaming-no-flicker.test.ts in webviews/: 0 findings (base: the unused-var warning and 7 TS errors).
• scripts/web-lint.sh: passed. python3 scripts/cmux-next/prepush-gate.py: no new violations.

Observed / Not run

• Observed: the commands above, on this VM.
• Not run: the browser test itself (a webviews browser lane runs only in CI or a cloud VM, never on a fleet Mac).

Changelog

none

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Summary by cubic

Fixes type-checking and lint errors in the no-flicker agent pane test so the webviews lint passes on this base.

• Removes the unused outer Box type; each page.evaluate body declares its own.
• Types the onAnimation parameter as globalThis.Event, since the file's own Event type (the acpmux event) shadowed the DOM Event and caused 7 TypeScript errors.

No test logic changes.