fix: measure nested transitions before applying their starting styles

[61021]

Fixes #18421 (regression from #16035, which re-broke the #4784 scenario).

Since #16035 the dummy delay animation runs with fill: 'forwards', so an intro's t = 0 styles (for slide, height: 0) apply the moment the transition is created and stay applied until the real animation starts a frame later. Transition functions of other elements in the same batch run after that and measure a collapsed DOM. With nested intros (the TreeView from the issue), intro effects run deepest-first, so every ancestor's slide captures a height where its children are pinned at 0. On the issue's REPL every <ul> animates 0px -> 36px (the height of a single row) and then jumps to its real size when the animation finishes, which is the "children pop out" behavior from #4784.

The fix waits one microtask before applying the initial styles and creating the dummy, the same way deferred transitions (crossfade) already work in animate(). Everything in the batch measures the DOM first; the microtask still runs before the next paint, so elements never render unstyled and the #14732 behavior is preserved. Verified in a real browser that a delayed intro (delay: 300) stays held at height: 0 for the whole delay, including the first painted frame.

Most of the diff is the existing dummy/onfinish block moving into queue_micro_task; hiding whitespace shows the actual change.

Test plan

• new runtime-browser test transition-nested-measure: three nested slide|global intros, asserting the outermost animation's final keyframe equals the natural height (100px). I ran the same assertion against a real Chromium on both builds: 0px on main, 100px with the fix. The unit-test mock can't catch this (it applies keyframes synchronously and jsdom has no layout), which is presumably also why fix: transitions might render animated elements without animation styles applied for the duration of some rendering frames when they start #16035 landed without a test.
• pnpm test runtime-runes runtime-legacy: 5972 passed, pnpm check and pnpm lint clean

[pkg-svelte-dev]

Install the latest version of svelte from 8423f6c:

pnpm add https://pkg.svelte.dev/svelte/c/8423f6c2a509bfb4f5ff7504a7398a5968724cbb

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18647

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

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Latest commit: 8423f6c

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