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>

docs/SECURITY.md

@@ -1,5 +1,16 @@
 # Security operations guide
 
+## The daemon's gRPC API
+
+`secs_gemd`'s tool API is **unauthenticated by design** — authentication
+belongs to the transport. The defaults are safe (`127.0.0.1`), and the
+production recommendation is a Unix domain socket
+(`--grpc unix:///run/secs_gemd/api.sock`, file-permission protected, zero
+network surface). Never bind it to an interface reachable from the
+equipment LAN; if cross-host access is unavoidable, front it with the same
+stunnel pattern described below for HSMS.
+
+
 HSMS is plain TCP — no auth, no encryption.  That's what every fab
 tool ships and what every MES expects.  Security comes from the
 network layer around the HSMS socket; this doc has the concrete