docs: make the 1.0.0 changelog readable rather than dense

[fjmorant]

The live v1.0.0 release is already updated with the result. This PR brings CHANGELOG.md in line, since #502 made the release be the CHANGELOG entry — which means how the entry reads is how the release reads.

What was wrong

The 1.0.0 entry was a wall of text:

• tight lists — no blank lines between items, so GitHub renders them without paragraph spacing and everything runs together
• bullets running to eight lines, several in a row
• two bullets carrying three paragraphs each (INode, listProps), buried mid-list with no heading to find them by
• nothing to anchor the eye — no tables, no code blocks, no subheadings between the section headers

141 lines of unbroken prose bullets.

Same facts, restructured

Change	Why
The two multi-paragraph bullets become #### subsections	INode and listProps are navigable instead of buried
An At a glance table near the top	the four numbers most readers actually want, before any prose
listProps merge order becomes a table	it was a three-way precedence rule written as a paragraph
Both upgrade steps become code blocks	shorter than describing the edit, and harder to misread
Lists are loose throughout	items separate instead of running together

Now 181 lines with 2 tables, 2 code blocks and 4 subsections, so the section headers aren't the only structure.

Nothing lost

I grepped the rewritten section for every term the original used — 20,000-level, object-hash, IRenderedNode, listProps, TOP_LEVEL, paddingLeftIncrement, node-view, keyExtractor, IRow, initialNumToRender, _internalId, readonly INode — all still present, and the code fences and table header rows are balanced.

Also fixed a stray blank line that had made one item of the Fixed list render as a separate list.

Note for next time

prettier-check only covers src/**/*.{ts,tsx}, so Markdown is unformatted and hand-aligned tables survive as written. That is what we want here, but worth knowing the CHANGELOG has no automated formatting behind it.

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