fix(sim): keep legacy villagers and newborns alive

Restore pre-life-cycle snapshots with working-age villagers and keep dependents near the shared food stock. Add regression coverage for legacy saves, child return-home behavior, and newborn placement.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/world.rs

@@ -1121,9 +1121,8 @@ impl World {
                 continue;
             }
 
-            let location = (self.people[first_index].x, self.people[first_index].y);
             self.food -= 6;
-            self.spawn_person_at(location);
+            self.spawn_person_at(self.home);
             if let Some(partnership) = self
                 .partnerships
                 .iter_mut()
@@ -1181,6 +1180,9 @@ impl World {
             p.hunger = (p.hunger - 25.0).max(0.0);
         }
         if !p.stage.can_partner() {
+            if !at_home {
+                (p.x, p.y) = self.move_toward(pos, home);
+            }
             p.action = "growing up".to_string();
             self.people[i] = p;
             return;
@@ -1437,7 +1439,21 @@ impl World {
     }
 
     pub fn from_snapshot(snapshot: &str, fallback_seed: u64) -> Result<Self, String> {
-        let saved: WorldSnapshot = serde_json::from_str(snapshot).map_err(|e| e.to_string())?;
+        let snapshot_value: serde_json::Value =
+            serde_json::from_str(snapshot).map_err(|e| e.to_string())?;
+        let legacy_people = snapshot_value
+            .get("people")
+            .and_then(serde_json::Value::as_array)
+            .is_some_and(|people| {
+                !people.is_empty()
+                    && people.iter().all(|person| {
+                        person
+                            .as_object()
+                            .is_some_and(|person| !person.contains_key("stage"))
+                    })
+            });
+        let saved: WorldSnapshot =
+            serde_json::from_value(snapshot_value).map_err(|e| e.to_string())?;
         if saved.version != 1 {
             return Err(format!(
                 "unsupported world snapshot version {}",
@@ -1479,11 +1495,47 @@ impl World {
             calendar: saved.calendar,
         };
         world.calendar.normalize();
+        if legacy_people {
+            world.initialize_legacy_people();
+        }
         world.prune_partnerships();
         world.sync_entities();
         Ok(world)
     }
 
+    fn initialize_legacy_people(&mut self) {
+        for index in 0..self.people.len() {
+            let stage = if index < 3 {
+                LifeStage::YoungAdult
+            } else {
+                LifeStage::Adult
+            };
+            let duration_days = self.sample_stage_duration(stage);
+            let prior_stages = match stage {
+                LifeStage::YoungAdult => {
+                    [LifeStage::Toddler, LifeStage::Child, LifeStage::Teen].as_slice()
+                }
+                LifeStage::Adult => [
+                    LifeStage::Toddler,
+                    LifeStage::Child,
+                    LifeStage::Teen,
+                    LifeStage::YoungAdult,
+                ]
+                .as_slice(),
+                _ => &[],
+            };
+            let age_days: u64 = prior_stages
+                .iter()
+                .map(|&past_stage| self.sample_stage_duration(past_stage) as u64)
+                .sum();
+            let person = &mut self.people[index];
+            person.stage = stage;
+            person.stage_duration_days = duration_days;
+            person.stage_elapsed_ticks = 0;
+            person.age_ticks = age_days * self.calendar.ticks_per_day;
+        }
+    }
+
     fn rect(t: &[&str], i: usize) -> Option<(usize, usize, usize, usize)> {
         let n: Vec<i64> = (i..i + 4).filter_map(|k| t.get(k)?.parse().ok()).collect();
         if n.len() != 4 {
@@ -1851,7 +1903,7 @@ impl Drop for World {
 
 #[cfg(test)]
 mod tests {
-    use super::{DEFAULT_SCRIPT, DEFAULT_TICKS_PER_DAY, LifeStage, World};
+    use super::{DEFAULT_SCRIPT, DEFAULT_TICKS_PER_DAY, LifeStage, World, cheb};
 
     #[test]
     fn new_people_progress_through_every_stage_and_die() {
@@ -1907,6 +1959,8 @@ mod tests {
                 .count(),
             8
         );
+        world.people[0].x = 0;
+        world.people[0].y = 0;
 
         world.sync_entities();
         assert_eq!(
@@ -1930,6 +1984,39 @@ mod tests {
             .find(|person| person.stage == LifeStage::Toddler)
             .expect("fertile partner actors should eventually have a child");
         assert_eq!(toddler.age_ticks, 0);
+        assert_eq!((toddler.x, toddler.y), world.home);
+    }
+
+    #[test]
+    fn legacy_snapshots_restore_the_village_as_adults() {
+        let world = World::new(42);
+        let mut snapshot: serde_json::Value =
+            serde_json::from_str(&world.snapshot_json().unwrap()).unwrap();
+        for person in snapshot["people"].as_array_mut().unwrap() {
+            let person = person.as_object_mut().unwrap();
+            for field in [
+                "age_ticks",
+                "stage",
+                "stage_elapsed_ticks",
+                "stage_duration_days",
+                "partner_id",
+            ] {
+                person.remove(field);
+            }
+        }
+        snapshot.as_object_mut().unwrap().remove("partnerships");
+
+        let restored = World::from_snapshot(&snapshot.to_string(), 42).unwrap();
+
+        assert_eq!(restored.people.len(), 9);
+        assert_eq!(restored.people[0].stage, LifeStage::YoungAdult);
+        assert!(
+            restored
+                .people
+                .iter()
+                .skip(3)
+                .all(|person| person.stage == LifeStage::Adult)
+        );
     }
 
     #[test]
@@ -1948,6 +2035,21 @@ mod tests {
         );
     }
 
+    #[test]
+    fn children_return_home_instead_of_staying_away_from_food() {
+        let mut world = World::new(42);
+        world.food = 100;
+        let home = world.home;
+        let position = (home.0.saturating_add(5), home.1.saturating_add(5));
+        world.spawn_person_at(position);
+
+        world.tick_person(world.people.len() - 1);
+
+        let child = world.people.last().unwrap();
+        assert!(cheb((child.x, child.y), home) < cheb(position, home));
+        assert_eq!(child.action, "growing up");
+    }
+
     #[test]
     fn snapshot_restores_state_and_rng() {
         let mut original = World::new(42);