[A7med7x7]

Exam phase 2 of 6 (docs/exams.md): course exams.

Stacked on #53 (phase 1, the question bank). This PR targets that branch, so the diff shows only phase 2. Merge #53 first; GitHub then retargets this one to main.

What students get

• /courses/<course>/exam, linked from every course page that has questions ("Take a course exam", with the best score so far on that device).
• 20 questions drawn across all modules, in proportion to each module's size, with at least one from every module. Every attempt draws a fresh set in a new order, with shuffled options. For example, Discrete Mathematics splits 4 / 4 / 4 / 3 / 3 / 2 over its six modules.
• No feedback until you submit, like a real exam. A numbered palette shows which questions are answered and jumps between them. Submitting with gaps asks for confirmation first and counts the gaps as wrong.
• Exam conditions (optional): 90 seconds a question (30 minutes for 20), a visible countdown that turns orange in the last minute, screen-reader announcements at 5 and 1 minutes left, and an automatic submit at zero.
• Results: total score, a bar per module with "Review this module", and every missed question with your answer, the right answer, the explanation, and a link to re-read the lesson.
• Keyboard: number keys pick an answer, Enter moves on.
• Attempts are kept in localStorage (decoded:exams:v1, last 20 per course). Syncing them to accounts is phase 6.

How it's built

• src/lib/exams.ts: pure, seeded logic (allocate uses the largest-remainder method, plus buildExam and scoreExam) and the attempt storage.
• src/content/questionBanks.ts: loads a course's bank on demand, so lesson pages never download question banks (checked in the build output: each bank is its own chunk).
• src/pages/ExamPage.tsx: lazy-loaded route, registered before the lesson route. A test checks no lesson is ever named exam.

Also in this PR

Before stacking, I rebased #53 onto main, where a Computer Graphics quiz option had been reworded ("slants" → "leans sideways"). I carried that change into the graphics bank and re-checked all 259 questions against main's quiz files: identical.

Tests (37 new)

• src/test/exams.test.ts: allocation adds up, respects limits and gives every module a question; on every real course bank, an exam has the right size, no repeats, covers every module that has questions, and keeps each question's right answer through the shuffle; seeding is deterministic; scoring by module adds up; attempts storage keeps 20 and survives corrupt data.
• src/test/exam-page.test.tsx: a perfect 20/20 run, submitting with gaps (confirmation, then "No answer" for each and explanations shown), number keys, auto-submit when the timer runs out (fake timers), and a course with no questions yet.

Checked

npm run lint (0 errors), tsc (app and node), npm run test (52 files, 2,433 tests), npm run build. In a browser at 390 px, light and dark: course page entry, intro, a timed exam answered by keyboard, results, and a direct load of /courses/logic-design/exam. No page errors, no horizontal overflow. The selected answer was too subtle at first, so its number now gets a filled badge.

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