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/src/simulation/save_io.rs

@@ -143,6 +143,7 @@ pub fn save_to_file(path: &Path, world: &mut World) -> Result<(), SaveLoadError>
             ids.sort_by_key(|id| id.0);
             ids
         },
+        modifications: vec![],
     };
 
     let bytes = state
@@ -558,6 +559,7 @@ mod tests {
             template_references: TemplateReferenceMap::default(),
             triangle_states: vec![],
             open_doors: vec![],
+            modifications: vec![],
         };
         let bytes = bad_state.to_bytes().expect("serialize");
         let path = temp_path();