feat(daemon): Phase E — production hardening, complete

Exposure: --grpc default flipped from 0.0.0.0 to 127.0.0.1 (the API is
unauthenticated by design; auth belongs to the transport), Unix-domain-
socket support (--grpc unix:///run/secs_gemd/api.sock = zero network
surface), SECURITY.md documents the contract and ch42 gained a "Running it
in production" section (which also documents the HSMS-SS single-session
assumption).

Graceful shutdown: SIGTERM/SIGINT land on an asio::signal_set on the io
thread, which nudges grpc Shutdown with a 2s deadline (cancels open
Subscribe/WatchHealth streams); Wait() returns on the MAIN thread, which
stops the engine (rt->stop() joins the io thread, so it must not run on
it). Exit 0, journal-safe, the in-code TODO is gone. --spool-dir added so
host-bound events survive daemon restarts.

Observability: --metrics serves Prometheus gauges secsgem_link_selected /
secsgem_control_state / secsgem_spool_depth, wired via the Phase-0
add_link_observer/add_control_state_observer hooks + io-thread sampling.

Deployment: deploy/secs_gemd.service — hardened systemd unit (DynamicUser,
ProtectSystem=strict, StateDirectory for the spool, UDS for the API,
TimeoutStopSec aligned with the graceful-shutdown window).

Enforcement: tools/check_daemon_ops.sh proves all three operational claims
(unix-socket gRPC accepts, all gauges present on /metrics, SIGTERM -> exit
0 + clean-stop log) — green; wired into tools/run_interop.sh (now 11
steps) and CI. CI python-interop lane also gained the pyclient and
spool-restart steps, so every harness now runs in CI.

TODO sweep: the shutdown TODO is fixed; the four remaining TODOs (nested
list formats, C2-as-text, U8>2^63, CONNECTED link state) are deliberate
deferred edge cases, each marked in code with context.

Daemon suite re-verified green (175 assertions).

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

tools/run_interop.sh

@@ -16,7 +16,9 @@
 #   py-host       secsgem-py active host vs C++ secs_server     (31 checks)
 #   conformance   secs_conformance C++ host vs C++ secs_server  (47 checks)
 #   daemon        gRPC tool + secsgem-py host vs secs_gemd      (bridge proof)
+#   pyclient      published secsgem-client package vs secs_gemd  (13 checks)
 #   spool         spool persistence across a server restart
+#   daemon-ops    unix-socket gRPC + Prometheus gauges + graceful SIGTERM
 #   tshark        Wireshark HSMS dissector on a live capture
 #   secs4j        secs4java8 Java host vs C++ secs_server       (55 checks)
 #
@@ -119,6 +121,11 @@ note "spool: persistence across server restart"
 record spool $?
 compose stop server-spool >/dev/null 2>&1
 
+# ---- daemon operations: unix socket + metrics + graceful shutdown ------------
+note "daemon-ops: unix socket, Prometheus gauges, SIGTERM drains cleanly"
+compose run --rm builder bash tools/check_daemon_ops.sh
+record daemon-ops $?
+
 # ---- Wireshark dissector ------------------------------------------------------
 note "tshark: Wireshark HSMS dissector"
 compose run --rm builder bash interop/tshark_validate.sh