docs: update README with render delegate support and Cycles build notes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -9,7 +9,7 @@ A C++17 / Windows desktop application providing a **Maya Render Layers-style** U
 - **Layer Stack Management** — Create, reorder, mute/unmute sublayers with full undo support
 - **Scene Hierarchy** — Browse prim tree with per-type SVG icons; create, delete, rename prims; add/replace references
 - **Property Editor** — Maya Channel Box-style attribute editing with type-aware drag inputs, color pickers, and undoable transforms
-- **3D Viewport** — Hydra rendering via `UsdImagingGLEngine` into an offscreen FBO; switchable render delegates; grid overlay; selection bounding boxes; camera wireframes
+- **3D Viewport** — Hydra rendering via `UsdImagingGLEngine` into an offscreen FBO; switchable render delegates (Storm, Embree, Arnold, Cycles); grid overlay; selection bounding boxes; camera wireframes
 - **Multi-Viewport** — Single, horizontal split, vertical split, and quad layouts; per-tile camera and render settings; Space to maximize/restore
 - **Camera Control** — Free orbital camera (tumble/truck/dolly) or drive from USD camera prims; auto near/far clipping; frame selection; correct camera prim orientation for Y-up and Z-up stages
 - **Transform Gizmo** — Pure ImDrawList-based manipulator (translate/rotate/scale) in object or world space
@@ -25,6 +25,16 @@ A C++17 / Windows desktop application providing a **Maya Render Layers-style** U
 | [GLAD](https://glad.dav1d.de/) | — | OpenGL 3.3 core loader |
 | Python | 3.12 | Required by OpenUSD runtime (`python312.dll`) |
 
+### Optional Render Delegates
+
+| Delegate | Variable | Notes |
+|---|---|---|
+| **hdEmbree** | `HDEMBREE_USD_ROOT` | USD build with `PXR_ENABLE_EMBREE_PLUGIN=ON`; set `EMBREE_LOCATION` if embree DLLs are not in `HDEMBREE_USD_ROOT/bin` |
+| **hdArnold** | `HDARNOLD_ROOT`, `ARNOLD_LOCATION` | [arnold-usd](https://github.com/Autodesk/arnold-usd) install dir + MtoA root for `ai.dll` |
+| **hdCycles** | `WITH_CYCLES=ON` | Built from source under `third_party/cycles` via ExternalProject; requires MSVC |
+
+All delegate paths are pre-configured in `CMakePresets.json`.
+
 ## Build
 
 ### Prerequisites
@@ -41,10 +51,16 @@ cmake --build build --config Release
 cmake --install build --config Release
 ```
 
-The installed `App.exe` lives in `install/bin/`.
+The installed executable lives in `install/bin/`.
 
 **Presets** — `default` (Release + tests), `debug`, `release`, `no-tests`.
 
+### Building with Cycles
+
+`WITH_CYCLES=ON` is set in the default preset. The first build compiles Blender Cycles from `third_party/cycles` via ExternalProject — expect a significantly longer initial build. Subsequent builds are incremental.
+
+Cycles runtime dependencies (`embree4.dll`, `openvdb.dll`, `OpenImageDenoise.dll`, `tbb12.dll`, etc.) are automatically deployed to the output directory by a post-build step. CRT DLLs bundled inside the Cycles install are excluded to avoid version conflicts with the app's own MSVC runtime.
+
 ### Tests
 
 ```powershell
@@ -123,6 +139,7 @@ Use Kilo's `openspec-*` skills to streamline this workflow.
 
 - **Python 3.12** — OpenUSD requires `python312.dll` at runtime. CMake copies it to the output directory via `POST_BUILD` commands.
 - **USD Plugin Path** — The app scans `<exe_dir>/usd/` for `plugInfo.json` and registers plugins via `pxr::PlugRegistry`.
+- **Render delegate DLLs** — hdEmbree, hdArnold, and hdCycles each bring their own runtime dependencies (Embree, Arnold SDK, Cycles libs). CMake `POST_BUILD` steps deploy all required DLLs next to the executable automatically.
 
 ## Contributing