LSP: go-to-definition and hover on a reference block

[aslakhellesoy]

Follow-up to #104 (ADR 0016).

A reference block is currently inert in the editor beyond ordinary Markdown link rendering. Three things would earn their keep:

• Go-to-definition on the link → the referenced heading. Obvious, and cheap: references(doc) already resolves the target path and slug purely.
• Hover showing the resolved steps. This is the one that matters. ADR 0016 argues references are for setup the reader must see; a hover that resolves the chain and shows the steps inline keeps that true at the point of use, and keeps a deep chain readable despite itself.
• Diagnostics on the block for reference-not-found / reference-empty / reference-cycle. The planner already emits these with a span on the reference block, so they should surface as squiggles — worth confirming they already do via the existing diagnostic rail rather than assuming.

The workspace index has everything needed: buildWorkspaceIndex holds every parsed doc and the reference topology (WorkspaceIndex.workspace).

[aslakhellesoy]

Resolved by #113 (merged as d5fde84): see the ADR 0016 section "Open questions, and how each was decided" and reference/examples.mdx for the decision as documented.