fix(simulation): address PR #2 review findings

- Fix Cargo.toml edition 2024 → 2021 (Hoshe #1)
- Switch test runner to cargo-nextest in Makefile (Hoshe #2, D-030)
- Add debug_assert tick ordering enforcement in InputQueue::push (Hoshe #4)
- Add DayPhase Serialize/Deserialize derives (Tyre #6)
- Add day phase boundary comments clarifying half-open ranges (Hoshe #3)
- Add phase duration adjustability comment (Tyre #5)
- Document ObserverSnapshot planned fields as TODO (Tyre #1)
- Document entity_id as wire-format ID, not ECS Entity (Tyre #2)
- Change Relationship.target_name to target_id: u64 (Tyre #3)
- Clarify single app.update() is intentional boilerplate (Hoshe #8, Tyre #7)

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

server/src/simulation/input.rs

@@ -13,8 +13,16 @@ pub struct InputQueue {
 }
 
 impl InputQueue {
-    /// Add a new input to the queue
+    /// Add a new input to the queue.
+    /// Inputs must be pushed in tick order for deterministic processing.
+    /// Panics in debug builds if tick ordering is violated.
     pub fn push(&mut self, input: PlayerInput) {
+        debug_assert!(
+            self.queue.back().is_none_or(|last| last.tick <= input.tick),
+            "InputQueue: tick ordering violated (last={}, new={})",
+            self.queue.back().map_or(0, |last| last.tick),
+            input.tick,
+        );
         self.queue.push_back(input);
     }
 
@@ -74,4 +82,18 @@ mod tests {
         let inputs = queue.drain_for_tick(10);
         assert!(inputs.is_empty());
     }
+
+    #[test]
+    #[should_panic(expected = "tick ordering violated")]
+    fn push_rejects_out_of_order_in_debug() {
+        let mut queue = InputQueue::default();
+        queue.push(PlayerInput {
+            tick: 5,
+            action: PlayerAction::MoveNorth,
+        });
+        queue.push(PlayerInput {
+            tick: 2,
+            action: PlayerAction::MoveSouth,
+        });
+    }
 }