Start CMake modernization scaffold

docs/adr/0001-modernization-boundaries.md

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+# ADR 0001: Incremental Component Boundaries
+
+Status: accepted
+Date: 2026-05-31
+
+## Context
+
+PanoPainter currently has a flat `src/` layout with broad dependencies through
+`pch.h`, global singletons such as `App::I` and `Canvas::I`, OpenGL types in
+high-level painting/document headers, and duplicated platform source lists.
+The modernization work must retain existing behavior across Windows desktop
+and AppX, macOS, iOS, Android standard, Quest, Focus/Wave, Linux, and WebGL.
+
+## Decision
+
+Modernization will proceed incrementally. OpenGL remains the production
+renderer while component boundaries and tests are introduced. Vulkan, Metal,
+and WebGPU-related work must stay out of the production path until OpenGL
+parity tests exist.
+
+The target dependency direction is:
+
+```text
+pp_foundation
+  -> pp_assets
+  -> pp_paint
+  -> pp_document
+  -> pp_renderer_api
+  -> pp_renderer_gl
+  -> pp_paint_renderer
+  -> pp_ui_core
+  -> pp_panopainter_ui
+  -> pp_platform_*
+  -> panopainter_app
+```
+
+Pure component headers must not include platform SDK headers or graphics API
+headers. Temporary shims are allowed only when recorded in
+`docs/modernization/debt.md`.
+
+## Consequences
+
+- The first implementation steps are documentation, inventory, CMake skeleton,
+  diagnostics, and tests, not a renderer rewrite.
+- Existing project files remain until the shared CMake targets are validated.
+- Refactors should prefer additive compatibility layers before moving behavior.
+- Every extracted component must gain its own tests before the next component
+  boundary is extracted.
+