footnotes: mark the number links and side note numbers as decorations, so text exports drop them

[ewels]

Closes #2

With starlight-llms-txt set to drop .scb-deco, each footnote under a code block still came through with its back link (1. [1.](#scb-fnref-…)Needs uv on PATH.), because a.scb-footnote-num was not in the list of decorations. The number list of side annotations had the same problem: its scb-annotation-note-num span repeated the <ol> number (1. 1Runs it.).

Both are now in the decoration list in core.ts, so they get scb-deco and data-pagefind-ignore like the other decorations. The numbers still show on screen: scb-deco adds no styles. The text exports now read as plain numbered lists, and Pagefind no longer indexes the numbers.

• New test in test/decorations.test.ts: with the decorations removed, each footnote item and side note reads as the note text only. It fails without the fix.
• The starlight-llms-txt section of the plugin compatibility page now says the note lists stay as plain numbered lists.

I checked the other lists outside the code. The print list of annotations (scb-annotation-list) has plain items with no numbers or links. Callouts, popovers and footnote badges were already decorations.

pnpm lint, pnpm test, pnpm lint:docs and pnpm docs:build pass. docs/dist/llms-full.txt has no scb-fnref links now.

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