feat(docs): split v3 llms.txt into spec-compliant index and full dump

[asblanco]

Summary

Mirrors the v2 split (#2283) for the v3 docs. The previous /llms.txt route produced a content-dump shape that doesn't match llmstxt.org (which wants an H1 + blockquote summary + H2 sections + markdown link lists).

This PR:

• /llms.txt — spec-compliant index. One # Stacks H1, blockquote summary, 9 H2 sections (Brand, Copywriting, Develop, Accessibility, Foundation, Base, Forms, Components, Resources), and - [Title](url): description per page. HTML stripped from descriptions with the loop-until-stable pattern (CodeQL js/incomplete-multi-character-sanitization). ~15 KB / 89 entries.
• /llms-full.txt — full content dump. Same content the previous /llms.txt produced, just at the new filename, plus the empty-collection bug fix.

Note

Implementation notes

• The new /llms.txt reads markdown frontmatter via Vite's ?raw query rather than loading the compiled Svelte page module. This keeps the index independent of every page component's imports — a missing icon export in a leaf component can't break the index.
• Pages not in structure.yaml (e.g. the standalone changelog.md) are skipped from the index by design — the YAML is the site's nav source of truth and decides what's a "page" worth indexing.

Warning

Coordination
Once this deploys, stacks-mcp-server PR #10 flips the Stacks v3 source URL in documentation-fetcher.ts from /llms.txt to /llms-full.txt. That PR is the v3 counterpart to stacks-mcp-server PR #9, which does the same for v2.

Test plan

• Local dev server: curl http://localhost:5173/llms.txt returns 200 with spec shape (1 H1, 1 blockquote, 9 H2 sections, 89 link-list entries). Folder-index pages get trailing-slash URLs; plain .md pages don't.
• After deploy: curl https://stackoverflow.design/llms.txt returns the spec-shape index.
• After deploy: curl https://stackoverflow.design/llms-full.txt returns the full content dump.
• Confirm the follow-up MCP change can then index v3 successfully.

JIRA: STACKS-864

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