feat(simulation): add modifications data model stub (#567, D-112)

DLC entry point for future player construction system. Adds
Modification struct, ModificationType enum, and Modifications
component. Wired into SaveStateV1 with #[serde(default)] for
forward-compatible save format.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/tests/environmental_interaction.rs

@@ -418,6 +418,7 @@ fn minimal_save() -> SaveStateV1 {
         template_references: TemplateReferenceMap::default(),
         triangle_states: vec![],
         open_doors: vec![],
+        modifications: vec![],
     }
 }
 

server/tests/information_boundaries.rs

@@ -206,6 +206,7 @@ fn save_state_npc_kg_isolation() {
         template_references: Default::default(),
         triangle_states: vec![],
         open_doors: vec![],
+        modifications: vec![],
     };
 
     // Roundtrip: serialize → deserialize.