[A7med7x7]

Exam phase 4a (docs/exams.md): an Operating Systems question bank.

Stacked on #59 → #58 → #53. This PR targets #59's branch, so the diff shows only this phase.

Why

Operating Systems (17 lessons) had no quizzes and no questions, so its exam page said "no questions yet" and none of its material could reach the review queue. It's one of the two biggest gaps the exam plan called out; Advanced Algorithms is next.

What's in it

src/content/operating-systems/questions.ts: 67 questions, about 4 per taught lesson and 2 per lab, covering all 5 modules. Unlike the 259 migrated questions, every one sets lesson and section, so "Re-read" links in exams and reviews go straight to the heading that teaches the idea.

• Concepts: interrupts, DMA, dual mode, the timer, system calls and their parameters, policy vs mechanism, microkernels, process states, the PCB, fork(), IPC models, the bounded buffer's BUFFER_SIZE − 1, RPC stubs, race conditions, the three critical-section requirements, Peterson, spinlocks, semaphores, deadlock, dining philosophers, the convoy effect, aging, MLFQ, affinity, MMU, fragmentation, demand paging, copy-on-write, Belady, OPT, second chance, thrashing, working sets.
• Calculations, with fresh numbers rather than the lessons' own examples: FCFS and SJF average waiting time, exponential averaging, a Round Robin trace, base and limit, best fit, address bits, address translation, internal fragmentation, TLB and demand-paging effective access time, FIFO vs LRU page faults.
• Labs: >> vs >, pipes, start() vs run(), join(), synchronized methods, wait/notify.

How it was checked

• Every calculation was worked again by script (FCFS 4.33, SJF 3.67, EAT 120 ns and ≈16.2 μs, fragmentation 2,288 B, translation 6,072, FIFO 4 vs LRU 5, and so on).
• Claims were checked against the lesson text, e.g. BUFFER_SIZE − 1, the semaphore initial values, "smallest integer = highest priority", the hash table for inverted page tables. Wrong options are typical mistakes, and each explanation says why the tempting one is wrong.
• New test in question-bank.test.ts: every section must be a real heading in its lesson. I broke one on purpose to confirm the test catches it.
• exam-page.test.tsx: its "course with no questions yet" example moved from Operating Systems to Advanced Algorithms.
• npm run lint (0 errors), tsc, npm run test (54 files, 2,453 tests), build.
• In a browser: the OS course page links to the exam, which draws 20 questions from all 5 modules, scores them per module, and gives every miss a "Re-read" link. Six repeated exams answering the first option scored about 1 in 4, as shuffling should give.

Note

Operating Systems still has no module quiz lessons, so this bank feeds exams and review only. Adding quiz lessons that draw on it is noted in docs/exams.md as a possible follow-up.

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