Added 'concept' objects to 4 lessons in basic selectors module:
- Type + ID: Explains specificity boost from combining selectors
- Selector Lists: Explains OR logic and independent matching
- Universal Selector: Explains wildcard matching and descendant context
- Specificity: Explains cascade and point system for selector precedence
All concepts include beginner-friendly explanations (2-4 sentences) and ASCII diagrams showing how selectors match elements and resolve conflicts.