docs: add Karpathy-style STUDY-GUIDE.md (4-track Q&A, grounded in the code)

Synthesised from a 4-stream investigation (JS/TS, Playwright, architecture,
tooling/CI/kit). ~100 interview-style Q&A answered from this repo's real
snippets: language features, fixtures/locators/assertions, GRASP/SOLID/DRY
decisions, and the gate/CI/.claude machinery. First-principles, self-study.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Kseolis