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>

include/secsgem/gem/data_model.hpp

@@ -161,7 +161,21 @@ class EquipmentDataModel {
     std::string name;
     s2::Item value;
   };
-  using HostCommandHandler = std::function<HostCmdAck(const std::vector<CommandParam>&)>;
+
+  // Declarative host-command effect, loaded from YAML. The server
+  // dispatches a command by looking up the spec and (optionally) firing
+  // a CEID emit / setting an alarm after the S2F42 reply is sent.
+  struct CommandSpec {
+    HostCmdAck ack = HostCmdAck::Accept;
+    std::optional<uint32_t> emit_ceid;
+    std::optional<uint32_t> set_alarm;
+  };
+
+  struct CommandResult {
+    HostCmdAck ack = HostCmdAck::InvalidCommand;
+    std::optional<uint32_t> emit_ceid;
+    std::optional<uint32_t> set_alarm;
+  };
 
   // --- SVID ---------------------------------------------------------------
   void add_status_variable(StatusVariable sv);
@@ -191,9 +205,9 @@ class EquipmentDataModel {
   TimeAck set_time_string(const std::string& time_str);
 
   // --- Host commands ------------------------------------------------------
-  void register_command(const std::string& rcmd, HostCommandHandler handler);
-  HostCmdAck dispatch_command(const std::string& rcmd,
-                              const std::vector<CommandParam>& params) const;
+  void register_command(const std::string& rcmd, CommandSpec spec);
+  CommandResult dispatch_command(const std::string& rcmd,
+                                 const std::vector<CommandParam>& params) const;
   bool has_command(const std::string& rcmd) const;
 
   // --- Collection events --------------------------------------------------
@@ -247,7 +261,7 @@ class EquipmentDataModel {
   std::map<uint32_t, DataVariable> dvids_;
   std::map<uint32_t, EquipmentConstant> ecids_;
   std::int64_t time_offset_seconds_ = 0;
-  std::map<std::string, HostCommandHandler> commands_;
+  std::map<std::string, CommandSpec> commands_;
 
   std::map<uint32_t, CollectionEvent> ceids_;
   std::map<uint32_t, Report> reports_;