fix(exam): keep each question whole across page breaks (#29)

[agiacalone]

Closes #29.

Problem

Exam questions frequently split across a page boundary — a stem stranded at the bottom of a page with its answer choices pushed to the next. Beyond readability, on a proctored paper exam this invites "I never saw part (d)" disputes.

Fix (template-only, preamble)

references/reference_exam.tex:

• \qreserve reserve (default 5\baselineskip) at the top of every top-level question \item via an enumitem before-hook — if that much room isn't left, the whole question moves to the next page. Level-2 (choice) items are exempt (the enumerate,2 hook restores the plain \item), so only whole questions are space-reserved.
• Stem↔choices glue via \@beginparpenalty=9999.
• Widow/orphan damping (\clubpenalty/\widowpenalty = 10000).
• \raggedbottom so pages run short rather than stretch to force-fit a question.

\qreserve is tunable per exam.

Why it's safe

• Automatic — no per-question authoring change.
• Parser-safe — the authored \item ... \textit{(N pts)} pattern is untouched, so exam_pack form-splitting (_QSTART_RE / _PTS_RE) and outline parsing are unaffected. No Python change.
• No backfill — applies to exams authored from the template going forward; existing exam .tex are not modified (per maintainer direction).

Verification

• 40-question, four-choice stress build: 3 split questions with the pre-Exam layout: keep each question together (prevent mid-question page breaks) #29 template, 0 with this template — at the same page count (no space blow-up).
• reference_exam.tex compiles clean in both student and \answersmode key builds.
• Full test suite: 405 passed (template-only change; nothing Python touched).

Docs

• docs/design/exam-tex-format.md — new "Page-break discipline — questions stay whole" section (mechanism, \qreserve tuning, parser-safety note, verification).
• CHANGELOG.md — Unreleased entry.

🤖 Generated with Claude Code

https://claude.ai/code/session_011L61RXxaELhXPvbW2RBF5m