G: CemObjectStore (E120 Common Equipment Model)

Hierarchical object tree for equipment self-description.  Each object
carries a CemObjectType (Equipment / Subsystem / IODevice / Module /
MaterialLocation / Other), an optional parent_objid, and a flat
attribute map keyed by name (the wire shape S14F1 / F3 returns).

Operations covered:
  add(CemObject)        - dedup'd, validates parent exists
  get / has             - lookup by objid
  get_attr / set_attr   - E14 GetAttr / SetAttr semantics
  children(parent)      - tree traversal; empty parent = roots

The flat-map representation matches how E14 ObjectService traffic
addresses nodes (by OBJSPEC string).  Wiring S14F1/F2 GetAttr and
S14F3/F4 SetAttr to this store is a downstream commit; the data model
is what was missing.

Joins EquipmentDataModel alongside the other top-level stores.  Three
test cases cover hierarchical add+dedup, children() traversal, and
get/set/missing attribute semantics.

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

src/gem/cem_objects.cpp

@@ -0,0 +1,17 @@
+#include "secsgem/gem/store/cem_objects.hpp"
+
+namespace secsgem::gem {
+
+const char* cem_object_type_name(CemObjectType t) {
+  switch (t) {
+    case CemObjectType::Equipment:   return "Equipment";
+    case CemObjectType::Subsystem:   return "Subsystem";
+    case CemObjectType::IODevice:    return "IODevice";
+    case CemObjectType::Module:      return "Module";
+    case CemObjectType::MaterialLoc: return "MaterialLocation";
+    case CemObjectType::Other:       return "Other";
+  }
+  return "?";
+}
+
+}  // namespace secsgem::gem