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/npc/mod.rs

@@ -26,7 +26,8 @@ pub struct Relationships {
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Relationship {
-    pub target_name: String,
+    /// Wire-format entity ID of the relationship target (scales to 10K+ NPCs)
+    pub target_id: u64,
     pub kind: String,
     pub trust_level: f32,
 }