Add claude docs

.claude/platform-windows.md

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+# Platform -- Windows
+
+`DUSK_TARGET_SYSTEM`: `windows`  
+Source layer: `src/duskwindows/` (planned, does not exist yet)  
+Status: **Planned -- not yet implemented**
+
+---
+
+## Overview
+
+Windows desktop is a planned target. No source layer, CMake target file,
+or toolchain exists yet. The intended architecture closely mirrors Linux:
+SDL2 for windowing/input, desktop OpenGL for rendering, pthreads (via
+MinGW or MSVC pthreads shim) for threading.
+
+---
+
+## Expected macros (when implemented)
+
+| Macro | Expected |
+|-------|---------|
+| `DUSK_WINDOWS` | yes |
+| `DUSK_SDL2` | yes |
+| `DUSK_OPENGL` | yes |
+| `DUSK_DISPLAY_SIZE_DYNAMIC` | yes |
+| `DUSK_INPUT_KEYBOARD` | yes |
+| `DUSK_INPUT_POINTER` | yes |
+| `DUSK_INPUT_GAMEPAD` | yes |
+| `DUSK_PLATFORM_ENDIAN_LITTLE` | yes |
+| `DUSK_TIME_DYNAMIC` | yes |
+| `DUSK_THREAD_PTHREAD` | yes |
+
+---
+
+## Notes
+
+- Will be little-endian (x86-64 Windows).
+- Expected to share `src/dusksdl2/` and `src/duskgl/` with Linux and
+  Knulli; only a thin `src/duskwindows/` layer for OS-specific
+  functionality (save paths, system dialogs) should be needed.
+- Save files will likely live in `%APPDATA%` or a sibling `saves/`
+  directory.
+- No cross-compiler needed; MSVC or MinGW-w64 on Windows or a
+  cross-compile from Linux.
+
+Update this document when the Windows target is implemented.