feat(clients)+test(interop): C++ client + Java validation of the daemon (B7)

B7 — the daemon's HSMS face under the Java reference: Dockerfile.server now
bakes secs_gemd alongside secs_server (grpc deps in both stages), and
secs4j_validate.sh gains TARGET=gemd to point the 55-check secs4java8 suite
at the daemon instead. Result: 55/55 green. With secsgem-py already
validating both faces, byte-identical GEM between secs_server and secs_gemd
is now proven by both reference implementations, not inferred from shared
code. CI runs the daemon target as an extra step (image layers shared).

Second client — clients/cpp: a header-only C++ twin of the Python client
over the same proto. eq.set("ChamberPressure", 2.5) with bare literals
(integral/floating dispatch avoids variant ambiguity), get/fire/alarm/
clear, control_state/request_control_state/health, on("START", fn) +
listen()/listen_async()/stop() with auto-CompleteCommand, SecsGemError
carrying the daemon's message. cpp_mini_tool (~30 lines) mirrors the
Python mini_tool. Tested end-to-end over real loopback TCP against the
service inside secs_gemd_tests — now 4 cases / 141 assertions — including
set/get round-trips, error text, alarm-by-name into the model, health,
and the full HCACK-4 command loop with parameters.

(Build note: two grpc-heavy TUs at -O3 OOM even at -j2 on Docker Desktop;
built -j1. Known environment limitation, roadmap-documented.)

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

CMakeLists.txt

@@ -170,6 +170,15 @@ if(SECSGEM_DAEMON)
       ${PROTO_OUT}/secsgem/v1/equipment.grpc.pb.cc)
     target_include_directories(secs_gemd PRIVATE ${PROTO_OUT} ${Protobuf_INCLUDE_DIRS})
     target_link_libraries(secs_gemd PRIVATE secsgem PkgConfig::GRPCPP ${Protobuf_LIBRARIES})
+    # The header-only C++ client's worked example (clients/cpp).
+    add_executable(cpp_mini_tool
+      clients/cpp/examples/mini_tool.cpp
+      ${PROTO_OUT}/secsgem/v1/equipment.pb.cc
+      ${PROTO_OUT}/secsgem/v1/equipment.grpc.pb.cc)
+    target_include_directories(cpp_mini_tool PRIVATE
+      ${PROTO_OUT} ${Protobuf_INCLUDE_DIRS} ${CMAKE_SOURCE_DIR}/clients/cpp/include)
+    target_link_libraries(cpp_mini_tool PRIVATE PkgConfig::GRPCPP ${Protobuf_LIBRARIES} Threads::Threads)
+
     set(SECSGEM_DAEMON_BUILT TRUE)
     message(STATUS "secs_gemd daemon enabled (grpc++ ${GRPCPP_VERSION})")
   else()
@@ -266,10 +275,11 @@ add_test(NAME secsgem_tests COMMAND secsgem_tests)
 if(SECSGEM_DAEMON_BUILT)
   add_executable(secs_gemd_tests
     tests/test_daemon_service.cpp
+    tests/test_cpp_client.cpp
     ${PROTO_OUT}/secsgem/v1/equipment.pb.cc
     ${PROTO_OUT}/secsgem/v1/equipment.grpc.pb.cc)
   target_include_directories(secs_gemd_tests PRIVATE
-    ${PROTO_OUT} ${Protobuf_INCLUDE_DIRS})
+    ${PROTO_OUT} ${Protobuf_INCLUDE_DIRS} ${CMAKE_SOURCE_DIR}/clients/cpp/include)
   target_link_libraries(secs_gemd_tests PRIVATE
     secsgem PkgConfig::GRPCPP ${Protobuf_LIBRARIES} doctest::doctest)
   target_compile_definitions(secs_gemd_tests PRIVATE