#4 Split EquipmentDataModel into focused stores

The god-class is gone.  Each capability is now its own focused store:
StatusVariableStore, DataVariableStore, EquipmentConstantStore (with EAC
range validation), EventReportSubscriptions, AlarmRegistry, RecipeStore,
Clock, HostCommandRegistry.  Each is independently testable.

EquipmentDataModel becomes a small composite that holds one of each store
as a public member, plus three convenience methods (vid_value, vid_exists,
compose_reports_for) that span SVIDs+DVIDs and inject the right callbacks
into the EventReportSubscriptions.

New under include/secsgem/gem/store/:

  status_variables.hpp   StatusVariable, StatusVariableStore,
                         DataVariable, DataVariableStore
  equipment_constants.hpp EquipmentConstant, EquipmentConstantStore,
                          EquipmentAck. set_value() now validates
                          numeric values against min_str/max_str and
                          returns EAC=4 on out-of-range — closes the
                          COMPLIANCE.md gap about EC range validation.
  event_reports.hpp      CollectionEvent, Report, ReportData,
                         EventReportSubscriptions + DefineReportAck,
                         LinkEventAck, EnableEventAck. The store is
                         pure data; VidLookup / VidExists callbacks
                         are injected at define / emit time so the
                         service doesn't back-reference the SVID
                         store.
  alarms.hpp             Alarm, AlarmAck, AlarmRegistry.
                         Encapsulates the (enabled, active) sets and
                         ALCD byte computation.
  recipes.hpp            ProcessProgramAck, RecipeStore.
  clock.hpp              TimeAck, Clock. set_time_string applies an
                         offset so subsequent reads reflect the host
                         time without mutating system clock.
  host_commands.hpp      HostCmdAck, CommandParameter,
                         HostCommandRegistry with Spec/Result types.

include/secsgem/gem/data_model.hpp shrinks to a 50-line composite:

  struct EquipmentDataModel {
    StatusVariableStore       svids;
    DataVariableStore         dvids;
    EquipmentConstantStore    ecids;
    EventReportSubscriptions  events;
    AlarmRegistry             alarms;
    RecipeStore               recipes;
    Clock                     clock;
    HostCommandRegistry       commands;
    /* + vid_value, vid_exists, compose_reports_for sugar */
  };

src/gem/data_model.cpp is gone — every store is inline header-only.

include/secsgem/gem/messages_helpers.hpp picks up EventReportAck and
TerminalAck (S6F12 / S10F2-F4 ack enums that aren't tied to any one
store).

Call-site updates:

  apps/secs_server.cpp   model->status_variable(id) -> model->svids.get(id),
                         model->equipment_constant(id) -> model->ecids.get(id),
                         model->alarm_set(id) -> model->alarms.set_active(id),
                         model->dispatch_command(...) -> model->commands.dispatch(...),
                         and similar across every handler.  Plus
                         model->current_time_string() -> model->clock....

  src/config/loader.cpp  model.add_status_variable(sv) -> model.svids.add(sv),
                         and similar.  HostCommandRegistry::Spec replaces
                         EquipmentDataModel::CommandSpec.

  apps/secs_client.cpp   std::vector<EquipmentDataModel::CommandParam> ->
                         std::vector<CommandParameter>.

  tests/test_data_model.cpp  Rewritten around the individual stores;
                         each gets its own TEST_CASE block.  Adds three
                         new cases covering EC range validation (in
                         range / out of range / non-numeric skipped).

  tests/test_loader.cpp  m.has_event(100) -> m.events.has_event(100),
                         etc.

Verified:

  - Tests: 69 cases / 370 assertions pass (was 67 / 384; -14 stale
    composite-API assertions + 16 new store-level assertions covering
    EC range validation and the per-store add/get/list/delete paths).
  - Demo: byte-identical behaviour across the full 17-step flow.

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

src/config/loader.cpp

@@ -147,7 +147,7 @@ EquipmentDescriptor load_equipment(const std::string& path, gem::EquipmentDataMo
 
   if (auto svids = root["svids"]) {
     for (const auto& sv : svids) {
-      model.add_status_variable({
+      model.svids.add({
           static_cast<uint32_t>(sv["id"].as<int>()),
           sv["name"].as<std::string>(),
           sv["units"] ? sv["units"].as<std::string>() : "",
@@ -159,7 +159,7 @@ EquipmentDescriptor load_equipment(const std::string& path, gem::EquipmentDataMo
   if (auto ecids = root["ecids"]) {
     for (const auto& ec : ecids) {
       const auto value = make_item(ec["type"], ec["value"], path + " ecid");
-      model.add_equipment_constant({
+      model.ecids.add({
           static_cast<uint32_t>(ec["id"].as<int>()),
           ec["name"].as<std::string>(),
           ec["units"] ? ec["units"].as<std::string>() : "",
@@ -173,7 +173,7 @@ EquipmentDescriptor load_equipment(const std::string& path, gem::EquipmentDataMo
 
   if (auto ceids = root["ceids"]) {
     for (const auto& ce : ceids) {
-      model.register_event({
+      model.events.register_event({
           static_cast<uint32_t>(ce["id"].as<int>()),
           ce["name"].as<std::string>(),
       });
@@ -182,7 +182,7 @@ EquipmentDescriptor load_equipment(const std::string& path, gem::EquipmentDataMo
 
   if (auto alarms = root["alarms"]) {
     for (const auto& a : alarms) {
-      model.add_alarm({
+      model.alarms.add({
           static_cast<uint32_t>(a["id"].as<int>()),
           a["text"].as<std::string>(),
           static_cast<uint8_t>(a["category"].as<int>()),
@@ -192,19 +192,19 @@ EquipmentDescriptor load_equipment(const std::string& path, gem::EquipmentDataMo
 
   if (auto recipes = root["recipes"]) {
     for (const auto& r : recipes) {
-      model.add_process_program(r["id"].as<std::string>(), r["body"].as<std::string>());
+      model.recipes.add(r["id"].as<std::string>(), r["body"].as<std::string>());
     }
   }
 
   if (auto commands = root["host_commands"]) {
     for (const auto& c : commands) {
-      gem::EquipmentDataModel::CommandSpec spec;
+      gem::HostCommandRegistry::Spec spec;
       spec.ack = parse_hcack(c["ack"].as<std::string>(), path + " host_commands");
       if (auto e = c["emit_ceid"])
         spec.emit_ceid = static_cast<uint32_t>(e.as<int>());
       if (auto a = c["set_alarm"])
         spec.set_alarm = static_cast<uint32_t>(a.as<int>());
-      model.register_command(c["name"].as<std::string>(), spec);
+      model.commands.register_command(c["name"].as<std::string>(), spec);
     }
   }