docs(docs): add strategy, coding, and system design documentation hierarchy

docs/content/02-guides/01-coding/07-hierarchical/binary-search-tree.mdx

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+---
+title: 'Binary Search Tree'
+description: 'A sorted array in tree form: in-order traversal is sorted order.'
+sidebar:
+  order: 5
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::

docs/content/02-guides/01-coding/07-hierarchical/index.mdx

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+---
+title: 'Hierarchical'
+description: 'Recursive shape: every node roots a subtree. Solutions are recurrences.'
+sidebar:
+  order: 1
+  label: Introduction
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::
+
+## Patterns
+
+- [DFS on Trees](/guides/coding/hierarchical/tree-dfs) — Post/pre/in-order recursion; the answer composes from subtree answers.
+- [BFS on Trees](/guides/coding/hierarchical/tree-bfs) — Level-order traversal via a queue.
+- [Trie](/guides/coding/hierarchical/trie) — Character-indexed tree; shared prefixes are shared paths.
+- [Binary Search Tree](/guides/coding/hierarchical/binary-search-tree) — A sorted array in tree form: in-order traversal is sorted order.
+
+**Systems anchor:** Trie is a radix routing table / IPLD path resolution; the BST invariant is a B-tree index.

docs/content/02-guides/01-coding/07-hierarchical/meta.ts

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+import { defineMeta } from "blume";
+
+export default defineMeta({
+  order: 7,
+});

docs/content/02-guides/01-coding/07-hierarchical/tree-bfs.mdx

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+---
+title: 'BFS on Trees'
+description: 'Level-order traversal via a queue.'
+sidebar:
+  order: 3
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::

docs/content/02-guides/01-coding/07-hierarchical/tree-dfs.mdx

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+---
+title: 'DFS on Trees'
+description: 'Post/pre/in-order recursion; the answer composes from subtree answers.'
+sidebar:
+  order: 2
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::

docs/content/02-guides/01-coding/07-hierarchical/trie.mdx

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+---
+title: 'Trie'
+description: 'Character-indexed tree; shared prefixes are shared paths.'
+sidebar:
+  order: 4
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::