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 {