Add claude docs

.claude/platform-macos.md

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+# Platform -- macOS
+
+`DUSK_TARGET_SYSTEM`: `macos`  
+Source layer: `src/duskmacos/` (planned, does not exist yet)  
+Status: **Planned -- not yet implemented**
+
+---
+
+## Overview
+
+macOS desktop is a planned target. No source layer, CMake target file,
+or toolchain exists yet. The intended architecture mirrors Linux: SDL2
+for windowing/input, OpenGL (or Metal via MoltenVK/SDL2) for rendering.
+
+---
+
+## Expected macros (when implemented)
+
+| Macro | Expected |
+|-------|---------|
+| `DUSK_MACOS` | 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 (Apple Silicon and Intel x86-64).
+- Apple deprecated OpenGL on macOS in 10.14 (Mojave). The implementation
+  will need to either target the deprecated OpenGL path or use MoltenVK
+  (Vulkan-over-Metal) with an SDL2 OpenGL layer. This decision is
+  pending.
+- Save files will likely live in `~/Library/Application Support/`.
+- Expected to share `src/dusksdl2/` and `src/duskgl/` with Linux.
+- Toolchain: native Clang via Xcode Command Line Tools, or a
+  cross-compile from Linux with osxcross.
+
+Update this document when the macOS target is implemented.