test(markdown-ir): cover mdast recovered diagnostics

[dowdiness]

Summary

• Add focused MarkdownIR mdast export coverage for recovered-node diagnostics.
• Parse [text](\n, inject a synthetic diagnostic over the recovered [ span, and assert mdastRecovered.diagnostics carries that incoming diagnostic instead of the fallback recovery diagnostic.

Reuse check

• Reused existing parse_cst, experimental_markdown_ir_from_cst_with_diagnostics, @core.DiagnosticSet::single, and experimental_markdown_ir_to_mdast_json APIs.
• Checked existing recovered-node incoming diagnostic, raw-node incoming diagnostic, and mdastRaw export diagnostic tests; no new helpers or public API changes were needed.

Verification

• cd examples/markdown && NEW_MOON_MOD=0 moon fmt --check

• cd examples/markdown && NEW_MOON_MOD=0 moon check --deny-warn

• cd examples/markdown && moon test --release — 83 passed

• cd examples/markdown && NEW_MOON_MOD=0 moon info

• Checked .mbti diffs: none

• bash check-docs.sh was run for documentation changes. N/A: no documentation changes.

Documentation And ADR Checklist

• No Markdown files were added, moved, or removed; docs/README.md update not needed.
• No plan was completed, deleted, or archived; ADR/no-ADR plan closure note not needed.
• No public API, subsystem removal/deprecation, policy, or major decision change; ADR not needed.
• Does not change likely next tasks for coding agents; memory update not needed.