Add design and product documentation

DESIGN.md

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+# Design — Lineform Studio
+
+## Direction
+
+Strict Swiss/brutalist editorial system: black and white, large indexed typography, architectural imagery, hard rules, and disciplined asymmetry.
+
+## System
+
+- Preserve the existing grid and semantic tokens; avoid soft SaaS cards and decorative gradients.
+- Use index numbers, captions, measured whitespace, and full-bleed project media.
+- Motion behaves like editorial sequencing, never spectacle.
+- Mobile keeps project metadata adjacent to its image and maintains keyboard-visible focus.
+
+## Intentional patterns
+
+One-pixel borders, side rules, and indexed tabs are deliberate parts of the architectural system and may be detector-waived with this rationale.