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

docs/content/02-guides/01-coding/04-contiguous-linear/index.mdx

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+---
+title: 'Contiguous Linear'
+description: 'O(1) index access, order is information.'
+sidebar:
+  order: 1
+  label: Introduction
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::
+
+## Patterns
+
+- [Two Pointers](/guides/coding/contiguous-linear/two-pointers) — Converging/parallel index walk that prunes the O(n^2) pair space using order.
+- [Sliding Window](/guides/coding/contiguous-linear/sliding-window) — Two pointers plus incremental state between them.
+- [Prefix Sum](/guides/coding/contiguous-linear/prefix-sum) — Precomputed cumulative state for O(1) range queries.
+- [Binary Search](/guides/coding/contiguous-linear/binary-search) — Halving over any monotonic predicate, not just sorted arrays.
+- [Matrices](/guides/coding/contiguous-linear/matrices) — 2D index arithmetic; often a graph in disguise.
+
+**Systems anchor:** Prefix sum is a materialized view; sliding window is stream processing with an eviction policy.