Present a semantic changelog

[hweihwang]

In one sentence: the Changes view that makes the model's output readable, plus the public factory other apps can call.

What and why

The model from #9043 produces descriptors nobody can read. This PR turns them into a view people can actually use — and gives other apps a way in.

What's in this PR

• The Changes view: records grouped under the section heading they occurred in, labelled by kind, each with its own before/after preview
• A public factory, window.OCA.Text.createMarkdownContentComparison, so other apps can mount a comparison
• Stable selection by descriptor ID, an explicit "no rendered differences" state, and a hide-formatting-only filter

The Changes view, shown through the Collectives dialog from nextcloud/collectives#2697 — Text ships no standalone host. Text owns the model and views; Collectives owns the dialog.

Design notes

• The changelog is the default view — "what changed" is the usual question.
• The factory is a capability, not a version: consumers check for the function, not a version string.
• A group of three edits renders three previews. One merged preview lost two of the three.

Stack

Depends on: #9043. Followed by #9045. This draft is opened stacked — its diff narrows to its own commit once #9043 merges, and I'll rebase before review.

Verification

ts:check and lint pass; complete suite 1,820/1,820. Grouping, previews, filters and navigation are exercised by the live end-to-end suite recorded for the Collectives PRs.

AI assistance

Commits carry Assisted-by: Codex:gpt-5.6-sol; an independent review pass was run with Claude Opus 5. Description drafted with Hermes (deepseek-v4-pro) from my notes, then reviewed by me.

Related

• Show differences (added/removed/changed text) when checking past versions collectives#599
• Version difference extraction #4422