fix(codegen): drop grpcio-tools, stop committing generated code

The codegen only ever needed a protoc binary, but every entry point installed
grpcio-tools to get one. That drags in the grpcio C extension, which has no
Windows/ARM64 wheel and falls back to building from source there, so the ARM64
job died with "Failed building wheel for grpcio" -> "protoc not found".

tools/codegen_toolchain.py resolves the toolchain instead: protoc from $PROTOC,
PATH, a cache, grpc_tools when already installed, or a pinned checksum-verified
protoc release unpacked into .codegen-tools/; protobuf and pyyaml from the
calling interpreter or a cached virtualenv it re-execs into (distro Pythons
refuse `pip install` under PEP 668). All four build scripts and all three CI
jobs are now just `python tools/run_codegen.py`, with no pip lines around it.

tools/config_metadata_pb2.py was the one generated file checked into git. The
orca.* option extensions are now read out of the descriptor set, which already
carries config_metadata.proto via --include_imports, so nothing is generated
into the tree -- and the protobuf>=6.33.5,<7 CI pin goes away with it, since it
only existed to satisfy gencode's hard ValidateProtobufRuntimeVersion check.
Generated C++ verified byte-identical under upb and the pure-Python protobuf
runtime (what win/arm64 installs), and under both grpc_tools' and standalone
protoc.

Also fixed:
- build_release_macos.sh still passed -DPython3_EXECUTABLE=<codegen venv>,
  pointing the bundled *embed* interpreter at the codegen environment -- the
  same confusion fae4b124 fixed on the CMake side.
- Tab.cpp #includes TabLayout_generated.cpp but had no dependency on
  codegen_config, so an incremental build after a .proto edit could compile it
  while the file was being rewritten. libslic3r already had this guard.
- ConfigCodegen.cmake now probes with `codegen_toolchain.py --check` (which
  never downloads or installs), prefers a host interpreter over the embed one,
  and lets a fresh clone generate at configure time instead of erroring out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/slic3r/CMakeLists.txt

@@ -831,6 +831,16 @@ else ()
 endif ()
 target_include_directories(libslic3r_gui PRIVATE Utils ${CMAKE_CURRENT_BINARY_DIR})
 
+# Tab.cpp #includes GUI/generated/TabLayout_generated.cpp, so it must not be compiled
+# while the codegen is rewriting it (same contract as libslic3r's generated includes).
+if(TARGET codegen_config)
+    add_dependencies(libslic3r_gui codegen_config)
+    set_source_files_properties(
+        "${CMAKE_CURRENT_SOURCE_DIR}/GUI/Tab.cpp"
+        PROPERTIES OBJECT_DEPENDS "${CONFIG_GENERATED_SOURCES}"
+    )
+endif()
+
 if (WIN32)
     target_include_directories(libslic3r_gui SYSTEM PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/../../deps/WebView2/include)
     target_link_libraries(libslic3r_gui Advapi32)