ui: Improve performance when streaming (#25225)

ui: Improve performance when streaming (#25225)

* ui: Improve performance when streaming

* ui: build sibling info map in branching utils

Moves the node map and sibling map construction from the
.by block into buildSiblingInfoMap() in branching.ts.

The map is built once per structural change and only read
afterwards, so it does not need SvelteMap reactivity. Keeping
the construction in plain TypeScript fixes the
svelte/prefer-svelte-reactivity lint error and groups the
branching logic where it already lives.

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Co-authored-by: Pascal <redacted>