Deepen Trees & Heap, add full Graph fundamentals-through-advanced pass

Trees (Set 06): traversal toolkit (TR.1a) + five basics warm-ups (TR.1b);
theory expanded with the TreeNode model and traversal-picker table.

Heap (Set 07): heapq toolbox (H.1a) + Last Stone Weight warm-up (H.1b);
theory expanded with sift-up/sift-down engine code and a heap-vs-BST-
vs-sorted-list comparison.

Graphs (Set 09): fundamentals/representations/DFS+BFS toolkit prelude
(GR.1a-c), four new types — cycle detection, bipartite check, MST,
Bellman-Ford & Floyd-Warshall (GR.6a-d) — and an advanced-connectivity
+ decision-tree/cheat-sheet appendix (GR.8-9). Cross-references Set 11
for Kruskal/DSU instead of duplicating it.

Author: DeepCodeWork