Scaffold the LLD Lab and HLD Lab as sibling pages

Two new single-file apps sharing the DSA lab's design system and
drill machinery (dashboard, spaced re-design ladder, flashcard/quiz
deck with persistent misses, journal, nav filter, step-player runner):

- lld/ — LLD atlas (L0-L7), the machine-coding interview script with
  the extension test as the graded moment, requirement->pattern
  signal router, 6 seed flashcards
- hld/ — HLD atlas (H0-H12), the 45-minute timeboxed script, latency
  numbers + QPS/storage estimation arithmetic, requirement->component
  signal router, 6 seed flashcards

Cross-linked three ways (DSA <-> LLD <-> HLD); main lab masthead now
links both. Sets arrive one at a time via DESIGN-PROMPT.md with
staged model-answer reveals in interview order.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: DeepCodeWork