Add collapsed "In plain English" layer under all 21 theory sections

Every theory section keeps its compact senior version and gains an
expandable "In plain English — the same idea, slower" block right
below it, collapsed by default: analogy-first rewrites (caterpillar
window, highway-milestone prefixes, tournament-bracket two pointers,
waiting-for-someone-taller stack, NO->YES strip binary search,
treasure-hunt lists, org-chart recursion, emergency-room heap,
wall-calendar intervals, cave-and-chalk graphs, breadcrumb maze
backtracking, party-circles union-find, index-card DP, restaurant
kitchen design, seat-renumbering matrices, and more).

Seven new diagrams inside the blocks: prefix milestones, the
caterpillar window, two-pointer dead zone, one-way linked list,
org-chart reports, pruned backtracking wing, and the LCS grid cell.
Figures: 14 -> 21. All 41 headless checks pass.

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

Author: DeepCodeWork