feat(markdown): add toMarkdown to limel-markdown

[civing]

Claude on behalf of Civing

@coderabbitai summary

Closes #4301. Part of Lundalogik/crm-insights-and-intelligence#392.

What

limel-markdown gains a toMarkdown() method: its value with every whitelisted custom element replaced by the markdown that element stands for, for targets that cannot render the element — a paste into another application, say.

An element opts in by implementing the new MarkdownRepresentable interface (toMarkdown(): Promise<string>). The viewer detects the method on the rendered instances by duck typing, so it imports nothing from any consumer, and only whitelisted elements are ever asked — they are the only custom elements the sanitizer lets into the DOM, and the whitelist is checked explicitly as well.

Why

The AI Assist copy action puts the answer's markdown on the clipboard. Custom elements in it — <limebb-object-chip limetype="person" objectid="1234"> — come out as raw markup in plain text and are stripped entirely from the rich flavour. The rendered chip already knows the person's name and URL; this lets the copy path ask it. The consumers (building-blocks for the chip, crm-components for the copy handler) follow in their own PRs once this is released.

The rich text editor's serializer, which writes the tag itself so its markdown round-trips, is unchanged: that is the storage form, this is the export form.

How

• substituteCustomElements(source, root, whitelist) pairs each written occurrence of a whitelisted tag with the rendered elements of that tag, by order, and splices in what each returns. Three written forms are recognised (self-closing, open through closing tag, open alone). When a tag's written and rendered counts disagree, every occurrence of that tag is removed rather than risk pairing wrongly. Elements without the method contribute their light-DOM text when written with a closing tag and nothing otherwise, since without one the parser nests the following text inside the element and it is already in the source.
• textChanged() keeps its render as a promise so toMarkdown() can wait for the elements to exist before asking them.
• 13 spec tests on the substitution rules; 3 browser e2e tests with a custom element defined in the page.

Review:

• Commits are atomic
• Commits have the correct type for the changes made
• Commits with breaking changes are marked as such (none — purely additive, the interface is @alpha)

Browsers tested:

(Check any that applies, it's ok to leave boxes unchecked if testing something didn't seem relevant.)

Windows:

• Chrome
• Edge
• Firefox

Linux:

• Chrome
• Firefox

macOS:

• Chrome (headless Chromium via the e2e suite)
• Firefox
• Safari

Mobile:

• Chrome on Android
• iOS

🤖 Generated with Claude Code

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[github-actions]

Documentation has been published to https://lundalogik.github.io/lime-elements/versions/PR-4300/