fix(kyojin): Isolate JIT and runtime toolchains

Allow a pinned JIT SDK without rebuilding PyTorch or the extension.
Keep runtime device bitcode unchanged: exporting TheRock bitcode into
the serving process crashes CLR 7.2 on a basic tensor operation.

Disable unsupported expandable segments and check all five HIP kernel
modules, with an optional GPU matmul and module-loading regression test.

README.md

@@ -95,6 +95,30 @@ nix run git+https://git.millerson.name/alex/millerson-overlay.nix.git#mcp-gatewa
 nix profile install git+https://git.millerson.name/alex/millerson-overlay.nix.git#mcp-gateway
 ```
 
+### Kyojin JIT Toolchain
+
+Kyojin's serving wrappers use the packaged ROCm compiler by default. Supply a
+pinned SDK to compile HIP JIT kernels with another compiler without rebuilding
+PyTorch or the HIP extension:
+
+```nix
+kyojin.override { jitRocmSdk = myJitSdk; }
+```
+
+The SDK must provide `bin/hipcc` and matching `amdgcn/bitcode`. Its compiler
+wrapper must set `HIP_DEVICE_LIB_PATH` to that bitcode **in the compiler
+subprocess only**, along with any required `ROCM_PATH` and Nix C++ toolchain
+flags. Keep the compiler wrapper's real path inside the SDK prefix so upstream's
+compiler provenance check recognizes it.
+
+The serving process keeps PyTorch's ROCm device libraries. Setting TheRock's
+Clang 23 bitcode globally makes the ROCm 7.2 runtime crash on a basic PyTorch
+matrix multiplication. The serving wrappers also disable unsupported expandable
+allocator segments. `nix build .#kyojin.tests.jit` checks compilation of five HIP
+kernel modules without requiring a GPU; `packages/kyojin/check-jit.py --gpu`, run
+with the package's Python environment and serving variables, additionally checks
+PyTorch matrix multiplication and module loading through its existing HIP runtime.
+
 ### nftablesbuilder Runtime Notes
 
 `nftablesbuilder` is a web interface for managing nftables. It consists of a