build: record physical plugin split experiment

docs/plugin_physical_split_experiment.md

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+# Infini Ops Physical Split Experiment
+
+This note records a local experiment for splitting a device/operator plugin outside the main `InfiniOps` repository while still building one `libinfiniops.so`.
+
+## Layout
+
+The external plugin root was created under `/tmp/infini-ops-physical-split/plugins`. The plugin directory was `/tmp/infini-ops-physical-split/plugins/external-cpu-add` and contained:
+
+- `plugin.json`
+- `plugin.cmake`
+- `ops/add/add.h`
+
+`ops/add/add.h` was copied from `src/native/cpu/ops/add/add.h` to simulate a physically split operator implementation.
+
+The manifest kept source paths relative to the external plugin directory:
+
+```json
+{
+  "name": "external-cpu-add",
+  "kind": "device",
+  "contract_version": 1,
+  "devices": ["cpu"],
+  "depends": [],
+  "cmake_entry": "plugin.cmake",
+  "source_roots": ["ops"],
+  "operator_roots": ["ops"],
+  "device_headers": {"cpu": "native/cpu/device_.h"},
+  "test_devices": {"cpu": "cpu"}
+}
+```
+
+The CMake entry registered resolved external paths for build/codegen consumption:
+
+```cmake
+infini_ops_register_device(
+    NAME external-cpu-add
+    CMAKE_ENTRY plugin.cmake
+    DEVICES cpu
+    SOURCE_ROOTS "/tmp/infini-ops-physical-split/plugins/external-cpu-add/ops"
+    OPERATOR_ROOTS "/tmp/infini-ops-physical-split/plugins/external-cpu-add/ops"
+    DEVICE_HEADERS cpu=native/cpu/device_.h
+    TEST_DEVICES cpu=cpu)
+
+target_compile_definitions(infiniops PUBLIC WITH_CPU=1)
+find_package(OpenMP REQUIRED COMPONENTS CXX)
+target_link_libraries(infiniops PRIVATE OpenMP::OpenMP_CXX)
+```
+
+## Commands
+
+Configure with the external plugin root and generated operator call instantiations enabled:
+
+```sh
+cmake -S . -B build/physical-split-external-add \
+  -DINFINI_OPS_PLUGINS=external-cpu-add \
+  -DINFINI_OPS_PLUGIN_ROOTS=/tmp/infini-ops-physical-split/plugins \
+  -DGENERATE_PYTHON_BINDINGS=OFF \
+  -DGENERATE_OPERATOR_CALL_INSTANTIATIONS=ON
+```
+
+Build the main library:
+
+```sh
+cmake --build build/physical-split-external-add --target infiniops --parallel 2
+```
+
+## Result
+
+The configure step loaded the external plugin and generated wrappers successfully:
+
+```text
+-- infini_ops plugins: `external-cpu-add`
+-- infini_ops plugin devices: `cpu`
+-- Generating wrappers - done
+```
+
+The generated registry pointed at the external operator root:
+
+```json
+{
+  "plugins": ["external-cpu-add"],
+  "devices": ["cpu"],
+  "source_roots": ["/tmp/infini-ops-physical-split/plugins/external-cpu-add/ops"],
+  "operator_roots": ["/tmp/infini-ops-physical-split/plugins/external-cpu-add/ops"],
+  "device_headers": {"cpu": "native/cpu/device_.h"},
+  "test_devices": {"cpu": "cpu"}
+}
+```
+
+Generated sources included the external implementation header directly:
+
+```cpp
+#include "/tmp/infini-ops-physical-split/plugins/external-cpu-add/ops/add/add.h"
+```
+
+The build completed and produced `build/physical-split-external-add/src/libinfiniops.so`. Symbol inspection found `Operator<infini::ops::Add, Device::Type::kCpu, 0>` and generated `Add` call instantiation symbols in that library.
+
+## Findings
+
+- Physical source split is viable for header-only operator implementations with the current build-time plugin mechanism.
+- The external plugin can keep `plugin.json` paths relative for contract validation, while `plugin.cmake` registers resolved paths for CMake/codegen.
+- The external plugin is responsible for platform-specific compile definitions and link libraries, such as `WITH_CPU=1` and `OpenMP::OpenMP_CXX` in this experiment.
+- The current generated code uses absolute includes for external operator headers. This works for local builds, but it is not ideal for installed build trees or reproducible generated artifacts. A future hardening step should add a stable external include-root mapping instead of embedding `/tmp/...` paths.
+- v1 still cannot add a new `Device::Type`; the external plugin reused the existing `cpu` device.