StaffML: link each question to recommended textbook reading (topic → chapter)

Idea

When a StaffML question is shown, offer a "want to learn more about this?" pointer back into the textbook — a go-deeper link tied to the question's topic, not an answer key. The textbook is where StaffML's content is sourced, so this closes the loop from practice back to the book (Vol 1 + Vol 2). Future versions could broaden to other sources; v1 is book-only.

Key finding — the plumbing is already dormant

A Resource type and a details.resources field already exist in the question schema, the TS types, and the corpus bundle — but 0 of the ~10,700 questions populate it and nothing renders it. Footer.tsx documents the reason it was parked: "Per-question book links are deferred until mlsysbook.ai URLs stabilize." This issue is to un-defer it.

Approach (low-effort, high-coverage)

Don't hand-author a reference per question. Every question already carries a topic (one of 87 curated) and competency_area (one of 13). Map the 87 topics → chapters once and derive each question's reference automatically at build time.

• Source of truth: a single topic_chapter_map.yaml (~87 rows: topic → primary chapter + optional "also see").
• Vault build joins topic → chapter and emits book_refs into the summary bundle (renders synchronously, no extra fetch).
• details.resources stays available for per-question overrides later.

URL stability (the original blocker — resolved)

The live chapter URL pattern is stable and verified (HTTP 200): https://mlsysbook.ai/vol{N}/contents/vol{N}/{chapter}/{chapter}.html Link at chapter granularity for v1 — section anchors carry auto-generated hash suffixes that regenerate on rebuild. Add a build-time link-checker so a mapped chapter that doesn't exist fails the build (turns "wait until URLs stabilize" into "URLs are enforced valid").

Pedagogy

Show the reading after the attempt, not before — a "Learn more" card below the revealed solution. Optionally, on a wrong answer, surface the prerequisite topic's chapter (the taxonomy already encodes prerequisite edges).

Phasing

1. topic_chapter_map.yaml + build join + chapter-level "Learn more" card + link-checker.
2. Prerequisite-on-wrong-answer; per-question resources overrides.
3. Bidirectional chapter → practice links; multi-source (papers/docs).

Filed as a future enhancement; a first-pass 87-row topic→chapter draft already exists to seed step 1.