#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>

include/secsgem/gem/store/recipes.hpp

@@ -0,0 +1,49 @@
+#pragma once
+
+#include <cstdint>
+#include <map>
+#include <optional>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace secsgem::gem {
+
+enum class ProcessProgramAck : uint8_t {
+  Accept = 0,
+  PermissionNotGranted = 1,
+  LengthError = 2,
+  MatrixOverflow = 3,
+  PpidNotFound = 4,
+  ModeUnsupported = 5,
+  PerformanceError = 6,
+};
+
+class RecipeStore {
+ public:
+  void add(std::string ppid, std::string body) {
+    by_ppid_.insert_or_assign(std::move(ppid), std::move(body));
+  }
+  std::optional<std::string> get(const std::string& ppid) const {
+    auto it = by_ppid_.find(ppid);
+    if (it == by_ppid_.end()) return std::nullopt;
+    return it->second;
+  }
+  std::vector<std::string> list() const {
+    std::vector<std::string> out;
+    out.reserve(by_ppid_.size());
+    for (const auto& [k, _] : by_ppid_) out.push_back(k);
+    return out;
+  }
+  ProcessProgramAck remove(const std::string& ppid) {
+    if (!by_ppid_.count(ppid)) return ProcessProgramAck::PpidNotFound;
+    by_ppid_.erase(ppid);
+    return ProcessProgramAck::Accept;
+  }
+  std::size_t size() const { return by_ppid_.size(); }
+
+ private:
+  std::map<std::string, std::string> by_ppid_;
+};
+
+}  // namespace secsgem::gem