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]

server/src/npc/mood.rs

@@ -47,12 +47,14 @@ pub enum NpcMood {
     /// Low stress, positive recent context — settled and cooperative.
     Content,
     /// Observing unusual or off-script behavior — targeted wariness.
+    /// Not reachable from `derive_mood()` — set externally by observation pipeline.
     Suspicious,
     /// Recent positive player interaction within memory window.
     Warm,
     /// Stress at or above threshold — confrontational or withdrawn.
     Hostile,
     /// Actively engaged in a scheduled activity — task-focused.
+    /// Not reachable from `derive_mood()` — set externally by activity scheduler (#101).
     Focused,
 }
 
@@ -145,7 +147,9 @@ pub fn derive_mood(
     // 2. Anxious: above 60% of threshold.
     // Guard: skip if threshold == 0 (divide-by-zero equivalent — entity
     // has no tolerance and is already Hostile from rule 1).
-    if threshold > 0 && current_stress * 100 >= threshold * ANXIOUS_STRESS_NUMERATOR {
+    if threshold > 0
+        && (current_stress as i32) * 100 >= (threshold as i32) * (ANXIOUS_STRESS_NUMERATOR as i32)
+    {
         return NpcMood::Anxious;
     }
 

server/src/npc/relationships.rs

@@ -141,8 +141,8 @@ impl RelationshipGraph {
     }
 
     /// Get all entities who have feelings about a target.
-    /// Full scan — use for event detection, not per-tick queries.
-    pub fn who_knows(&self, target: &StableId) -> Vec<(&StableId, &RelationshipEdge)> {
+    /// O(N) full scan of all edges — use for event detection, not per-tick queries.
+    pub fn who_knows_full_scan(&self, target: &StableId) -> Vec<(&StableId, &RelationshipEdge)> {
         self.edges
             .iter()
             .filter(|((_, t), _)| t == target)
@@ -374,7 +374,7 @@ mod tests {
             make_edge(RelationshipKind::Family, 8),
         );
 
-        let knowers = graph.who_knows(&target);
+        let knowers = graph.who_knows_full_scan(&target);
         assert_eq!(knowers.len(), 3);
     }