refactor(test): restructure test kernels into test/kernel/[platform]/

- Create test/kernel/ with top-level dispatcher Makefile
- Move platform-agnostic headers to test/kernel/include/
- Create test/kernel/nvidia/ with clean CUDA kernels (no #ifdef)
- Create test/kernel/du/ with clean DU kernels (no #ifdef)
- Update unittest/device_api and perf/device_api Makefiles to use new paths
- Split test_nvshmem_adaptor.cu into kernel (.cu) + harness (.cpp)
- Move nvshmem_state_sync.cc to test/unittest/shmem_adaptor/
- Add KERNEL_DIR/KERNEL_PLATFORM/KERNEL_OBJ_DIR to test/make.inc

Old test/device_api/ directory kept temporarily for reference until
build verification passes on GPU nodes.

Author: MC952-arch

test/kernel/Makefile

@@ -0,0 +1,22 @@
+# test/kernel/Makefile
+# Dispatches kernel compilation to the active platform subdirectory.
+
+include ../make.inc
+
+KERNEL_PLATFORM_DIR := $(CURDIR)/$(KERNEL_PLATFORM)
+
+.PHONY: all clean
+
+all:
+	@$(MAKE) -C $(KERNEL_PLATFORM_DIR) all \
+		USE_SHMEM=$(USE_SHMEM) SHMEM_HOME=$(SHMEM_HOME) \
+		FORCE_DEFAULT_PATH=$(FORCE_DEFAULT_PATH)
+
+clean:
+	@$(MAKE) -C $(KERNEL_PLATFORM_DIR) clean
+
+# Pass through any specific target (device_api.o, dlink_ir.o, etc.)
+%:
+	@$(MAKE) -C $(KERNEL_PLATFORM_DIR) $@ \
+		USE_SHMEM=$(USE_SHMEM) SHMEM_HOME=$(SHMEM_HOME) \
+		FORCE_DEFAULT_PATH=$(FORCE_DEFAULT_PATH)

test/kernel/du/Makefile

@@ -0,0 +1,47 @@
+# test/kernel/du/Makefile
+# DU platform kernel compilation for test binaries.
+
+include ../../make.inc
+
+# Propagate FORCE_DEFAULT_PATH
+ifeq ($(FORCE_DEFAULT_PATH), 1)
+ADAPTOR_DEFINE += -DFLAGCX_FORCE_DEFAULT_PATH
+endif
+
+PLATFORM_INCLUDES := -I$(PLATFORM_IR_DIR)
+
+ALL_INCLUDES := $(FLAGCX_INCLUDES) \
+    -I$(PROJECT_ROOT)/flagcx/adaptor/include/device_api \
+    -I$(PROJECT_ROOT)/bindings/ir \
+    -I$(PROJECT_ROOT)/test/kernel/include \
+    $(PLATFORM_INCLUDES) \
+    $(CCL_INCLUDE) \
+    $(DEVICE_INCLUDE)
+
+NVCC_RDC_FLAG := -c
+
+.PHONY: all clean device_api.o device_ir.o
+
+all: $(OBJDIR)/device_api.o $(OBJDIR)/device_ir.o
+
+device_api.o: $(OBJDIR)/device_api.o
+device_ir.o: $(OBJDIR)/device_ir.o
+
+$(OBJDIR)/device_api.o: device_api.cu
+	@mkdir -p $(OBJDIR)
+	@echo "Compiling device_api.cu (DU)"
+	@$(DEVICE_COMPILER) -std=c++17 $(DEVICE_COMPILER_GENCODE) $(NVCC_RDC_FLAG) -o $@ $< \
+		$(ALL_INCLUDES) -DCOMPILE_KERNEL $(ADAPTOR_DEFINE)
+
+$(OBJDIR)/device_ir.o: device_ir.cu
+	@mkdir -p $(OBJDIR)
+	@echo "Compiling device_ir.cu (DU)"
+	@$(DEVICE_COMPILER) -std=c++17 $(DEVICE_COMPILER_GENCODE) $(NVCC_RDC_FLAG) -o $@ $< \
+		$(ALL_INCLUDES) -DCOMPILE_KERNEL $(ADAPTOR_DEFINE)
+
+# DU does not use SHMEM device-link targets
+dlink.o dlink_api.o dlink_ir.o:
+	@echo "DU: no device-link needed (no SHMEM support)"
+
+clean:
+	@rm -rf $(BUILDDIR)