feat(errors): structured Q-15-1 diagnostic for duplicate crossref identifiers (bd-rr6qzcvu)

[cscheid]

Summary

A duplicate crossref identifier (e.g. two cells both #| label: fig-charts) was reported as a bare, code-less, span-less line:

Error: duplicate crossref identifier `fig-charts`

It rendered as plain text only because to_text gates the ariadne box on has_any_location, and the diagnostic carried no location — even though duplicate_id_diagnostic already received both occurrences' SourceInfo and discarded them with a TODO.

This PR (two staged commits) wires the spans through and clears the real instance.

Stage 1 — the structured diagnostic

• New crossref subsystem (number 15) + code Q-15-1 "Duplicate Crossref Identifier" in the catalog.
• duplicate_id_diagnostic rebuilt with DiagnosticMessageBuilder: with_code + with_location(second) (underlines the duplicate) + add_detail_at("first defined here", first) + problem + hint. Still error-level.
• Docs page docs/errors/crossref/Q-15-1.qmd (new crossref/ subsystem dir).
• Refreshed the stale CONTRIBUTING-ERRORS.md subsystem table (12–15).

The diagnostics already flow ctx.diagnostics → render_output.diagnostics, rendered against the document's source context, so the located form renders with ariadne automatically:

Error: [Q-15-1] Duplicate crossref identifier
   ╭─[ dup.qmd:9:1 ]
 5 │ ![First chart](a.png){#fig-charts}    ── first defined here
 9 │ ![Second chart](b.png){#fig-charts}   ── The crossref identifier `fig-charts` is defined more than once. …
  ℹ Give one of the targets a different `label:` (or `#id`).

On the real docs/ render the diagnostic spans both multi-line code cells in figures.qmd.

Stage 2 — clear the instance

The "Subcaptions" example in figures.qmd showed the same figure in a Jupyter tab and a Knitr tab, both labeled fig-charts → a duplicate id in the rendered DOM. Relabeled to fig-charts-jupyter / fig-charts-knitr. No @fig-charts reference exists anywhere in docs/, so nothing else changes. q2 render docs/ goes from 1 error, 27 warnings to 27 warnings (no error).

Tests

• Catalog presence test for Q-15-1 (subsystem crossref).
• Transform test duplicate_id_diagnostic_is_coded_and_located: code + primary location + a located detail.

Verification

• cargo nextest run --workspace — 10,289 passed.
• cargo xtask verify (Rust + WASM + hub-client) — all steps passed.
• End-to-end: minimal fixture and the real docs/ render both show the boxed [Q-15-1] underlining both sites; after the relabel, docs/ renders with 0 errors.

Follows the same pattern as #318 (Q-5-6/Q-5-7 resource-copy diagnostics).

Plan: claude-notes/plans/2026-06-19-crossref-duplicate-id-diagnostic.md

🤖 Generated with Claude Code