docs: revise CoreJS Interview #1 — reduce scope, add OOP & HTTP

[dzmitry-varabei]

Summary

• 17 → 10 theoretical sections (merged fragmented sections, removed trivial topics, added OOP & HTTP)
• 14 → 7 coding tasks (removed trivial, antipattern, and duplicates with Interview Float exercise #2)
• Added OOP/Classes/Prototypes — 12 hours of Stage 1 curriculum were not tested in either interview
• Added HTTP Basics — 7 hours of Stage 1 curriculum (Week 10) were not tested anywhere
• Added recommended interview flow — warm-up → theory → coding → wrap-up with timing
• Added priority markers — core / (advanced) / (optional) so mentors can adapt to student level
• Added difficulty grading for coding tasks — Easy (~5 min) / Medium (~10 min) / Hard (~15 min)
• Expanded Web Storage & Clean Code from 1 to 3 points each (practical questions instead of "list abbreviations")
• Fixed markdown bug — "Advanced Functions" section was accidentally nested inside "Closures Advanced" bullet list
• Estimated time: ~135 min → ~68-83 min (target: 45-90 min)

Old version preserved as interview-basic-coreJS-legacy.md. Detailed rationale in interview-basic-coreJS-changelog.md.

Related: #1907 (Interview #2 revision)

Test plan

• Review interview-basic-coreJS-changelog.md — every change has a detailed justification
• Compare with interview-basic-coreJS-legacy.md — no content lost without explanation
• Cross-check with Interview Float exercise #2 (interview-corejs.md) — no topic overlap, no gaps
• Verify time budget fits 45-90 minute target

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