Stop the Document View lazy-body tests timing out under suite load

[dovvnloading]

Problem

This was already on main. PR #416 split the Document View's markdown body
behind a React.lazy import and updated its tests to await the result. Those
awaits use testing-library's default 1000ms.

That is enough when the file runs alone. It is not enough when the full suite
runs:

	result
DocumentViewPanel.test.tsx alone	45 passed
Full suite, 3 runs	2 failed

CI passed on #416 and on the merge, so this would have started failing
unrelated pull requests rather than announcing itself.

Why the wait is long

Only in the test environment. In a built bundle the lazy body is a prebuilt
chunk; under vitest it is the first transform of react-markdown plus six
remark/rehype plugins, with 93 test files competing for the worker pool while
it happens. Nothing about the panel is slow.

Change

Wait properly, rather than reach for fake timers or stub the import away —
stubbing it would stop the tests exercising the thing #416 actually changed.
One named constant at the four call sites that await the lazy body, with the
measurement written down beside it.

Verified both directions

A longer timeout is exactly the kind of change that can quietly turn a test
into a no-op, so I checked it hasn't:

• Five consecutive full-suite runs: 2,197 passed every time (was 2 failures
  in 3 runs).
• With the panel's body wired to never render, seven tests in the file fail
  and the suite goes red — the wait has not made them vacuous.

Test plan

• npm run check green.
• Python suite: 3,285 passed, 20 skipped.

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