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/bridge/types.rs

@@ -5,7 +5,10 @@
 use serde::{Deserialize, Serialize};
 
 /// The ONLY data structure crossing the client-server boundary (D-020)
-/// Contains all information visible to the observer at a given tick
+/// Contains all information visible to the observer at a given tick.
+///
+/// TODO: Planned fields — fog/visibility data, ambient sound events,
+/// internal monologue triggers, HUD state (D-020 expansion).
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ObserverSnapshot {
     /// Simulation tick when this snapshot was produced
@@ -17,6 +20,8 @@ pub struct ObserverSnapshot {
 /// A visible entity in the simulation
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct VisibleEntity {
+    /// Wire-format entity identifier — NOT a bevy ECS Entity.
+    /// Stable across serialization for client-server boundary (D-020).
     pub entity_id: u64,
     pub x: f32,
     pub y: f32,