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.

packages/kyojin/package.nix

@@ -6,6 +6,8 @@
   fetchFromGitHub,
   makeWrapper,
   rocmPackages,
+  # Override only the serving toolchain; torch and the extension keep their ROCm.
+  jitRocmSdk ? null,
 }:
 
 let
@@ -170,6 +172,7 @@ let
   };
 
   python = py.python.withPackages (_: [ library ]);
+  jitSdk = if jitRocmSdk == null then rocmToolkit else jitRocmSdk;
 in
 stdenv.mkDerivation {
   inherit src version;
@@ -203,8 +206,9 @@ stdenv.mkDerivation {
       makeWrapper ${python}/bin/python $out/bin/kyojin-serve-$model \
         --set PYTORCH_ROCM_ARCH ${rocmArch} \
         --set CC ${stdenv.cc}/bin/cc \
-        --set EXL3_ROCM_SDK ${rocmToolkit} \
+        --set EXL3_ROCM_SDK ${jitSdk} \
         --set HIP_DEVICE_LIB_PATH ${rocmPackages."rocm-device-libs"}/amdgcn/bitcode \
+        --set EXL3_EXPANDABLE_SEGMENTS 0 \
         --prefix PATH : ${lib.makeBinPath [ stdenv.cc ]} \
         --prefix PYTHONPATH : ${src}/gr \
         --add-flags "${src}/tools/$model/serve.py"
@@ -216,6 +220,23 @@ stdenv.mkDerivation {
   passthru = {
     inherit python rocmArch;
     pythonPackage = library;
+    jitRocmSdk = jitSdk;
+    tests.jit =
+      pkgs.runCommand "kyojin-jit-check"
+        {
+          EXL3_ROCM_SDK = jitSdk;
+          # CLR/COMGR also reads this variable. Newer bitcode crashes torch
+          # matmul; a custom hipcc must select its own bitcode in its subprocess.
+          HIP_DEVICE_LIB_PATH = "${rocmPackages."rocm-device-libs"}/amdgcn/bitcode";
+          EXL3_EXPANDABLE_SEGMENTS = "0";
+          PYTHONPATH = "${src}/gr";
+          KYOJIN_REQUIRE_SDK = lib.boolToString (jitRocmSdk != null);
+        }
+        ''
+          export HOME="$TMPDIR"
+          ${python}/bin/python ${./check-jit.py}
+          touch "$out"
+        '';
     category = "AI Inference";
     updateScript = [
       "nix-update"