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>

interop/secs4j_validate.sh

@@ -10,6 +10,10 @@
 # filesystem.  Wired into CI via .gitea/workflows/ci.yml.
 #
 # Usage: bash interop/secs4j_validate.sh
+#        TARGET=gemd bash interop/secs4j_validate.sh
+#            -> runs the same 55 checks against secs_gemd's HSMS face
+#               (the daemon must be byte-identical GEM to secs_server,
+#               since both sit on register_default_handlers).
 # Exit codes:
 #   0 — every check the harness defines passed
 #   1 — one or more checks failed
@@ -41,13 +45,22 @@ docker build -t secsgem-secs4j-interop -f interop/secs4j/Dockerfile interop/secs
 docker network rm "$NET" >/dev/null 2>&1 || true
 docker network create "$NET" >/dev/null
 
-echo "starting secs_server..."
+echo "starting ${TARGET:-server} (secs4j peer)..."
 # --name doubles as the DNS hostname on the user-defined network, so
 # the harness reaches it as "secs4j-interop-server:5000".
-docker run -d --rm \
-    --name "$SERVER_NAME" \
-    --network "$NET" \
-    secsgem-secs4j-server >/dev/null
+if [ "${TARGET:-server}" = "gemd" ]; then
+    docker run -d --rm \
+        --name "$SERVER_NAME" \
+        --network "$NET" \
+        --entrypoint /usr/local/bin/secs_gemd \
+        secsgem-secs4j-server \
+        --port 5000 --grpc 0.0.0.0:50051 --config-dir /app/data >/dev/null
+else
+    docker run -d --rm \
+        --name "$SERVER_NAME" \
+        --network "$NET" \
+        secsgem-secs4j-server >/dev/null
+fi
 
 # Give the server a moment to bind.
 sleep 1