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_state.rs

@@ -43,6 +43,7 @@ use crate::content::template::{TemplateOwnership, TemplateReferenceMap, Triangle
 use crate::knowledge::graph::KnowledgeGraph;
 use crate::knowledge::registry::StableEntityId;
 use crate::knowledge::types::StableId;
+use crate::simulation::modification::Modification;
 use crate::npc::{
     CombatCapability, Contentment, DailyRoutine, InformationInventory, JobPerformance, Npc,
     PersonalityTraits, Relationships, Secret, SecretSeverity, SkillSet, TellSystem,
@@ -99,6 +100,11 @@ pub struct SaveStateV1 {
     /// for deterministic serialization (D-010).
     #[serde(default)]
     pub open_doors: Vec<StableId>,
+    /// Player-placed modifications to map chunks (D-111/D-112, #567).
+    /// DLC stub — empty in v0.1. The save slot exists so future construction
+    /// DLC can populate it without a save format migration.
+    #[serde(default)]
+    pub modifications: Vec<Modification>,
 }
 
 /// Per-NPC state snapshot for `SaveStateV1`.
@@ -411,6 +417,7 @@ mod tests {
             template_references: TemplateReferenceMap::default(),
             triangle_states: vec![],
             open_doors: vec![],
+            modifications: vec![],
         }
     }
 
@@ -808,6 +815,7 @@ mod tests {
             template_references: TemplateReferenceMap::default(),
             triangle_states: vec![],
             open_doors: vec![],
+            modifications: vec![],
         };
 
         let bytes = save.to_bytes().expect("serialize");
@@ -828,4 +836,60 @@ mod tests {
         }));
         assert!(result.is_err(), "must panic without StableEntityId");
     }
+
+    // -----------------------------------------------------------------------
+    // Modifications stub round-trip (#567, D-111/D-112)
+    // -----------------------------------------------------------------------
+
+    #[test]
+    fn empty_modifications_roundtrips_in_save_state() {
+        // Acceptance (#567): empty modifications field survives save/load.
+        let state = minimal_save_state();
+        assert!(state.modifications.is_empty());
+
+        let bytes = state.to_bytes().expect("serialize");
+        let recovered = SaveStateV1::from_bytes(&bytes).expect("deserialize");
+        assert!(
+            recovered.modifications.is_empty(),
+            "empty modifications must roundtrip"
+        );
+
+        let bytes2 = recovered.to_bytes().expect("re-serialize");
+        assert_eq!(bytes, bytes2, "modifications roundtrip must be idempotent");
+    }
+
+    #[test]
+    fn populated_modifications_roundtrips_in_save_state() {
+        // Acceptance (#567): non-empty modifications field survives save/load.
+        use crate::simulation::modification::{ModificationType, Modification};
+
+        let mut state = minimal_save_state();
+        state.modifications = vec![
+            Modification {
+                position: TilePosition::new(10, 20, 0),
+                modification_type: ModificationType::Placeholder,
+                placed_at_tick: 500,
+            },
+            Modification {
+                position: TilePosition::new(3, 7, -1),
+                modification_type: ModificationType::Placeholder,
+                placed_at_tick: 1200,
+            },
+        ];
+
+        let bytes = state.to_bytes().expect("serialize");
+        let recovered = SaveStateV1::from_bytes(&bytes).expect("deserialize");
+        assert_eq!(
+            recovered.modifications.len(),
+            2,
+            "two modifications must survive roundtrip"
+        );
+        assert_eq!(recovered.modifications[0].position, TilePosition::new(10, 20, 0));
+        assert_eq!(recovered.modifications[0].placed_at_tick, 500);
+        assert_eq!(recovered.modifications[1].position, TilePosition::new(3, 7, -1));
+        assert_eq!(recovered.modifications[1].placed_at_tick, 1200);
+
+        let bytes2 = recovered.to_bytes().expect("re-serialize");
+        assert_eq!(bytes, bytes2, "modifications roundtrip must be idempotent");
+    }
 }