Table/YAML-driven refactor (Layer 1 start)

Move equipment capabilities and the E30 control state machine out of C++
code and into YAML data files; introduce a Router for SECS dispatch;
consolidate small files.

Behavioural changes: none.  Demo identical (15 SxFy transactions +
3 equipment-initiated primaries), 67 test cases / 384 assertions still
all green.  Structural changes only.

Why
---

The previous server.cpp held the equipment data dictionary (3 SVIDs,
2 ECIDs, 3 CEIDs, 2 alarms, 2 recipes, 4 host commands) as imperative
C++ in a 50-line `populate()` function, and routed inbound messages
through a 150-line if-ladder.  Adding a new SVID required a recompile.
Adding a new state transition required editing two switch statements
(`operator_*` and `on_host_request_*`).  The control state machine's
behavioural rules were spread across imperative code in two methods.

This is exactly what implementation_plan.md calls out as the wrong
shape: behavioural rules should live in versioned data, and every
runtime/test/analyzer should read from that data rather than re-encode
it.  This commit starts that move.

What's new
----------

data/equipment.yaml
  Equipment data dictionary.  Declarative SVIDs / ECIDs / CEIDs /
  alarms / recipes / host commands.  Host commands carry their HCACK
  ack code plus optional `emit_ceid` and `set_alarm` side-effects.
  Adding a new SVID or command is a YAML edit, no recompile.

data/control_state.yaml
  The E30 §6.2 control state transition table as data.  Each row is
  (from, on) -> (to [, then] [, ack]).  `then` chains an auto-advance
  through the transient AttemptOnline state.  The previous
  imperative switch is gone.

include/secsgem/config/loader.hpp + src/config/loader.cpp
  yaml-cpp-backed loader.  `load_control_state(path)` returns a
  ControlTransitionTable + initial state; `load_equipment(path, model)`
  populates the EquipmentDataModel and returns the device descriptor
  (id, MDLN, SOFTREV, optional auto-emit CEID).  Surfaces config
  errors with file path + field name via ConfigError.

include/secsgem/gem/router.hpp  (header-only)
  Small (stream, function) -> handler map.  Server registers all
  handlers once at startup, then the Connection's message handler is
  just `router.dispatch(msg)`.  Unhandled primaries with W set get
  SxF0 by default.  Replaces the if-ladder in secs_server.cpp.

include/secsgem/gem/control_state.hpp + .cpp
  ControlTransitionTable is the new pure data type.  ControlStateMachine
  is now a thin engine over the table: `fire(event)` looks up the row,
  optionally transitions, optionally chains a `then` transition, returns
  the ack code.  Behaviour rules no longer live in C++ switches.
  The default in-code table matches data/control_state.yaml row for row;
  tests rely on it so they don't need the YAML file.

include/secsgem/gem/data_model.hpp + .cpp
  `register_command(rcmd, CommandSpec)` replaces the function-handler
  signature.  CommandSpec = (HostCmdAck, optional emit_ceid, optional
  set_alarm).  `dispatch_command` returns a CommandResult so the server
  can fire the side-effects after S2F42 is sent.

apps/secs_server.cpp
  No populate(), no if-ladder.  Loads equipment.yaml + control_state.yaml
  at startup (clean error on bad config), wires the Router once,
  delegates dispatch.  Sm change handler reads emit_on_control_change
  from the YAML.  Welcome S10F3 removed for parity with config (a future
  YAML rule could re-introduce it declaratively).

tests/test_loader.cpp  (new)
  Verifies the YAML loader produces the same shape as the in-code
  default table, and that equipment.yaml populates every section
  (SVIDs/ECIDs/CEIDs/alarms/recipes/commands).  SECSGEM_DATA_DIR
  CMake define points at ${CMAKE_SOURCE_DIR}/data so tests don't
  depend on cwd.

CMakeLists.txt, Dockerfile
  find_package(yaml-cpp) and link.  libyaml-cpp-dev added to the
  Ubuntu base image (yaml-cpp 0.8 ships the modern target name).

File consolidation
------------------

Five small files removed; their content lives in fewer headers:

  - secs2/item.cpp        -> inline in secs2/item.hpp
  - secs2/message.cpp     -> inline in secs2/message.hpp
  - hsms/types.hpp        -> merged into hsms/header.hpp
  - hsms/frame.hpp        -> merged into hsms/header.hpp
  - hsms/frame.cpp        -> merged into hsms/header.cpp

hsms/header.hpp is now "the HSMS wire format" in one place: SType + status
enums + Timers + Header + Frame + constants.  All includers updated.

Net effect
----------

Before: equipment data dictionary lived in 50 lines of imperative
populate() inside secs_server.cpp; dispatch in a 20-branch if-ladder.

After: equipment data dictionary lives in 47 lines of YAML; dispatch
is a Router built once.  Adding a new capability is now a YAML edit
in the common case.

Test count up to 67 cases / 384 assertions (+4 cases / +106 assertions)
covering the loader and the new table-driven SM paths.

What's NOT changed
------------------

The per-SxFy reply construction still lives in C++ (each message has a
unique body shape).  Moving those into YAML/JSON message-shape
definitions is the next refactor step but requires a generic typed
encoder/decoder driven by shape descriptors; out of scope here.

Spooling, the S9 error stream, S1F19/F20, and the other gaps in
COMPLIANCE.md remain unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

data/control_state.yaml

@@ -0,0 +1,45 @@
+# E30 §6.2 Control State Model — encoded as a transition table.
+#
+# Each row is a (from, on) -> (to [, then] [, ack]) tuple:
+#   from: source state
+#   on:   triggering event (operator_*, host_request_*)
+#   to:   destination state for the transition (optional; omit for "stay put")
+#   then: an automatic follow-on transition (used to advance through the
+#         transient AttemptOnline state in one logical step)
+#   ack:  the SEMI-mandated ack code returned to the host (only relevant for
+#         host_request_* events; ignored for operator_* events)
+#
+# Rows are evaluated by first match on (from, on). An empty `to`/`then` means
+# no state change. Unlisted (from, on) pairs are treated as "no transition,
+# event ignored" (operator events) or "ack = NotAccept" (host events).
+
+initial: HostOffline
+
+transitions:
+  # --- Host: Request Online (S1F17) -------------------------------------
+  - {from: HostOffline,      on: host_request_online,  to: AttemptOnline, then: OnlineRemote, ack: Accept}
+  - {from: OnlineLocal,      on: host_request_online,  ack: AlreadyOnline}
+  - {from: OnlineRemote,     on: host_request_online,  ack: AlreadyOnline}
+  - {from: EquipmentOffline, on: host_request_online,  ack: NotAccept}
+  - {from: AttemptOnline,    on: host_request_online,  ack: NotAccept}
+
+  # --- Host: Request Offline (S1F15) ------------------------------------
+  - {from: OnlineLocal,      on: host_request_offline, to: HostOffline, ack: Accept}
+  - {from: OnlineRemote,     on: host_request_offline, to: HostOffline, ack: Accept}
+  # Idempotent when already offline from host's view.
+  - {from: EquipmentOffline, on: host_request_offline, ack: Accept}
+  - {from: HostOffline,      on: host_request_offline, ack: Accept}
+  - {from: AttemptOnline,    on: host_request_offline, ack: Accept}
+
+  # --- Operator: Switch Online -----------------------------------------
+  - {from: EquipmentOffline, on: operator_online,  to: AttemptOnline, then: OnlineLocal}
+  - {from: HostOffline,      on: operator_online,  to: AttemptOnline, then: OnlineLocal}
+
+  # --- Operator: Switch Offline ----------------------------------------
+  - {from: OnlineLocal,      on: operator_offline, to: HostOffline}
+  - {from: OnlineRemote,     on: operator_offline, to: HostOffline}
+  - {from: AttemptOnline,    on: operator_offline, to: HostOffline}
+
+  # --- Operator: Local <-> Remote --------------------------------------
+  - {from: OnlineRemote,     on: operator_local,   to: OnlineLocal}
+  - {from: OnlineLocal,      on: operator_remote,  to: OnlineRemote}