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/npc/interaction.rs

@@ -47,7 +47,7 @@ pub struct InteractionMemory {
     pub last_interaction_tick: u64,
     /// Notable events: walk-aways and confrontations.
     /// Bounded by `MAX_NOTABLE_EVENTS` — oldest entries dropped when full.
-    pub notable_events: Vec<InteractionEvent>,
+    pub notable_events: std::collections::VecDeque<InteractionEvent>,
 }
 
 /// Maximum number of notable events retained per NPC pair.
@@ -65,9 +65,9 @@ impl InteractionMemory {
     /// Append a notable event, dropping the oldest if at capacity.
     pub fn push_event(&mut self, event: InteractionEvent) {
         if self.notable_events.len() >= MAX_NOTABLE_EVENTS {
-            self.notable_events.remove(0);
+            self.notable_events.pop_front();
         }
-        self.notable_events.push(event);
+        self.notable_events.push_back(event);
     }
 
     /// Returns `true` if this is the first meeting (count == 0).
@@ -145,7 +145,7 @@ mod tests {
         });
         assert_eq!(mem.notable_events.len(), MAX_NOTABLE_EVENTS);
         assert_eq!(mem.notable_events[0].tick, 1); // tick=0 dropped
-        assert_eq!(mem.notable_events.last().unwrap().tick, 99);
+        assert_eq!(mem.notable_events.back().unwrap().tick, 99);
     }
 
     #[test]