Enrich 44 book pages with human, line-by-line code walkthroughs; add 6 new pages from the Kotlin repo (Dynamic Connectivity, Apply Substitutions, KD-Tree, How Many Rectangles Overlap, Streamer Leaderboard, Serialize N-ary Tree); weave in alternative implementations; close the coverage gap (91 uncovered files -> 0) and refresh the roadmap · arpanpathak/AdvancedAlgorithmPatterns@a13bc4c