100%/C: Limits Monitoring (S2F45–F48, E30 §6.21)

New LimitMonitorStore keyed by VID; each entry is a vector of
LimitDefinition (LIMITID + upper/lower deadband as arbitrary Items).
S2F45/F46 set, S2F47/F48 read.  VLAACK validates each VID exists.

Four new SxFy in the catalog; codegen handles the nested
list-of-(VID, list-of-LimitDefinition) shape.  LimitDefinition is
defined in store/limits.hpp and referenced as external_struct so the
data model and the message codecs share one type.

The actual "value crossed limit" detection + CEID emission is left to
the application's set_value path (E30 §6.21 leaves *how* the equipment
detects crossings up to the implementer).

COMPLIANCE.md: Limits Monitoring Additional capability flips ✅.
Tests: 80 cases / 465 assertions.

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

include/secsgem/gem/data_model.hpp

@@ -5,6 +5,7 @@
 #include "secsgem/gem/store/equipment_constants.hpp"
 #include "secsgem/gem/store/event_reports.hpp"
 #include "secsgem/gem/store/host_commands.hpp"
+#include "secsgem/gem/store/limits.hpp"
 #include "secsgem/gem/store/recipes.hpp"
 #include "secsgem/gem/store/spool.hpp"
 #include "secsgem/gem/store/status_variables.hpp"
@@ -26,6 +27,7 @@ struct EquipmentDataModel {
   Clock                     clock;
   HostCommandRegistry       commands;
   SpoolStore                spool;
+  LimitMonitorStore         limits;
 
   // Convenience: VID -> value lookup spanning SVIDs and DVIDs.
   std::optional<s2::Item> vid_value(uint32_t vid) const {

include/secsgem/gem/store/limits.hpp

@@ -0,0 +1,64 @@
+#pragma once
+
+#include <cstdint>
+#include <map>
+#include <utility>
+#include <vector>
+
+#include "secsgem/secs2/item.hpp"
+
+// E30 §6.21 Limits Monitoring.
+//
+// A limit definition attaches an upper/lower deadband pair to a VID under
+// some limit id.  Multiple definitions per VID are allowed (different
+// LIMITIDs can monitor different ranges).  Store is pure data; the actual
+// "value crossed limit" detection and CEID emission is the application's
+// job (typically wired into the SVID/DVID set_value path).
+
+namespace secsgem::gem {
+
+namespace s2 = secsgem::secs2;
+
+// Defined here (not in the generated messages.hpp) so the data model can
+// own the type and the codegen can reference it as external_struct.
+struct LimitDefinition {
+  uint8_t  limitid;
+  s2::Item upper;
+  s2::Item lower;
+};
+
+enum class LimitMonitorAck : uint8_t {  // S2F46 VLAACK
+  Accept = 0,
+  LimitsNotEqualToExisting = 1,
+  CannotChangeAttributes = 2,
+  LimitValueError = 3,
+  VidNotExist = 4,
+  VidNoLimitSupport = 5,
+};
+
+class LimitMonitorStore {
+ public:
+  void set_for_vid(uint32_t vid, std::vector<LimitDefinition> defs) {
+    by_vid_.insert_or_assign(vid, std::move(defs));
+  }
+  void clear() { by_vid_.clear(); }
+
+  std::vector<LimitDefinition> get_for_vid(uint32_t vid) const {
+    auto it = by_vid_.find(vid);
+    if (it == by_vid_.end()) return {};
+    return it->second;
+  }
+  std::vector<uint32_t> all_vids() const {
+    std::vector<uint32_t> out;
+    out.reserve(by_vid_.size());
+    for (const auto& [vid, _] : by_vid_) out.push_back(vid);
+    return out;
+  }
+  bool has(uint32_t vid) const { return by_vid_.count(vid) > 0; }
+  std::size_t size() const { return by_vid_.size(); }
+
+ private:
+  std::map<uint32_t, std::vector<LimitDefinition>> by_vid_;
+};
+
+}  // namespace secsgem::gem