Fix useLayoutEffect frame flicker

[sindresorhus]

Fixes #773

I'm not sure about it yet. It has some potential regressions in the tradeoffs. Feedback welcome.

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How it works now:

• On every interactive render to writable stdout, Ink does not write immediately.
• Instead it stores the latest interactive output and accumulates any new output in pendingInteractiveFrame.
• It schedules one microtask flush. In that flush it first settles throttled render work and runs flushSyncWork(), then writes one coalesced frame.
• Before external writes (useStdout/useStderr), waitUntilRenderFlush, clear, and unmount, Ink force-flushes any pending frame so ordering stays correct.

How this fixes #773:

• Previously only the first interactive frame was deferred. Later commits could still paint a pre-useLayoutEffect frame and then repaint, causing flicker.
• Now every interactive commit uses the defer-and-coalesce path, so mount/remount/update commits only paint the post-layout settled frame.
• <Static> still works because static chunks from deferred passes are accumulated and emitted once with the coalesced frame.

Tradeoffs:

• Output timing is microtask-delayed instead of immediate synchronous writes; callers that depend on flush completion should await waitUntilRenderFlush().
• Multiple commits in one tick may collapse into one frame, which removes flicker but hides intentional transient intermediate states.
• There is small per-frame overhead from queueMicrotask plus sync flush before write.
• Behavior differs between real writable streams (deferred path) and non-stream-like outputs used in some tests (sync path).
• Render timing logic is more complex, so regressions are easier to introduce without focused tests.