refactor: move CUTLASS ownership into sd.cpp

docs/build.md

@@ -111,7 +111,7 @@ cmake .. -DSD_CUDA=ON
 cmake --build . --config Release
 ```
 
-CUDA builds with the bundled patched GGML use the CUTLASS submodule by default
+CUDA extensions use sd.cpp's `thirdparty/cutlass` submodule by default
 for eligible INT8 GEMM and CuTe attention operations. If its headers are missing,
 CMake stops with an initialization hint; it does not download them automatically.
 Run `git submodule update --init --recursive` from the repository root before
@@ -121,14 +121,17 @@ To build with the existing CUDA implementations without CUTLASS, configure from
 the repository root with:
 
 ```shell
-cmake -S . -B build -DSD_CUDA=ON -DGGML_CUDA_CUTLASS_PATH:PATH=
+cmake -S . -B build -DSD_CUDA=ON -DSD_CUDA_CUTLASS_PATH:PATH=
 cmake --build build --config Release
 ```
 
-Alternatively, set `GGML_CUDA_CUTLASS_PATH` to another local CUTLASS source
+Alternatively, set `SD_CUDA_CUTLASS_PATH` to another local CUTLASS source
 directory (tested with 3.9.2). Existing CMake caches retain their configured path;
-pass `-UGGML_CUDA_CUTLASS_PATH` when reconfiguring to restore the bundled default.
-Non-CUDA builds do not require CUTLASS.
+pass `-USD_CUDA_CUTLASS_PATH` when reconfiguring to restore the bundled default.
+Existing `GGML_CUDA_CUTLASS_PATH` caches are migrated on first configuration:
+the old bundled path becomes `thirdparty/cutlass`, while custom paths and an
+empty disabled value are preserved. An explicitly set `SD_CUDA_CUTLASS_PATH`
+takes precedence. Non-CUDA builds do not require CUTLASS.
 
 ## Build with HipBLAS