fix(simulation): address PR #50 review — determinism, overflow, invariants

- Sort eligible NPCs by StableId for deterministic conversation pairing (D-010)
- Replace single_mut() with per-observer iteration (D-010 principle 3, D-027)
- Widen current_stress * 100 to i32 in mood derivation to prevent i16 overflow
- Promote line_interval to named constant LINE_INTERVAL_TICKS
- Add 4 Sprint 14 component invariants (MoodState, InteractionMemory,
  ActivityState+PathRequest exclusion, double-tagged tier detection)
- Attach MoodState to content spawn pipeline and gauntlet fixup
- VecDeque for InteractionMemory.notable_events (O(1) pop_front)
- Rename who_knows() to who_knows_full_scan() to communicate O(N) cost
- Add O(N^2) growth limit comment on conversation pair scan
- Document Suspicious/Focused as externally-set moods on enum variants

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

server/src/content/spawn.rs

@@ -114,6 +114,9 @@ fn spawn_npc(world: &mut World, profile: &types::NpcProfile, result: &mut SpawnR
     // Interaction history — drives Layer 2 situation activation (#325, D-028)
     entity_commands.insert(npc::interaction::InteractionMemory::default());
 
+    // Mood state — drives Layer 4 dialogue selection and monologue tone (#323)
+    entity_commands.insert(npc::mood::MoodState::default());
+
     // Axis 1: Want
     if let Some(want) = &profile.want {
         if let Some(kind) = parse_want_kind(&want.primary) {