fix(simulation): address PR #81 review — critical and high-priority issues

Critical fixes:
- storyteller: replace .expect() with guard + log in activation_pass (Hoshe #1)
- content/loader: validate inverted tile_bounds before iteration (Hoshe #C)
- save_io: persist ActivationState on save/load (Tyre #7)

High-priority fixes:
- storyteller: f64 intermediate for observation_time_ticks scoring (Hoshe #A)
- storyteller: deduplicate copresent entities before scoring (Hoshe #B)
- storyteller: skip activation_pass at tick 0 (Hoshe #G)
- storyteller: explicit .before(advance_tick) ordering (Tyre #9)
- save_state: insert EngagementRecord on NPC deserialize (Tyre #10)
- save_io: reset TriangleActivatedQueue + MovementHistoryBuffer on load (Tyre #8)
- debug: validate teleport target walkability (Hoshe #E)
- debug: reject SkipToContamination when tick past delay (Hoshe #F)
- debug: DebugEnabled defaults to cfg!(debug_assertions) (Hoshe #3)
- debug: log when response overwritten (Hoshe #2)
- content/loader: error on tile dimension mismatch (Hoshe #5)
- content/types: DistrictMeta.description optional (Hoshe #D)
- types: version docs updated to v18 (Tyre #1, #2, #3)

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

server/src/storyteller/mod.rs

@@ -382,7 +382,8 @@ impl Plugin for StorytellerPlugin {
                 Update,
                 activation_pass
                     .after(append_player_history)
-                    .after(tick_contamination_activation),
+                    .after(tick_contamination_activation)
+                    .before(crate::simulation::time::advance_tick),
             );
 
         tracing::debug!("StorytellerPlugin initialized");
@@ -448,7 +449,9 @@ pub fn tick_contamination_activation(
 fn compute_engagement_score(record: &EngagementRecord) -> f32 {
     let conv_score = record.conversation_count.min(CONVERSATION_CAP) as f32
         * ENGAGEMENT_WEIGHT_CONVERSATION;
-    let obs_score = record.observation_time_ticks as f32 * ENGAGEMENT_WEIGHT_OBSERVATION;
+    // Use f64 intermediate to avoid precision loss beyond ~16.8M ticks (2^24),
+    // where f32 can no longer distinguish adjacent integers.
+    let obs_score = (record.observation_time_ticks as f64 * ENGAGEMENT_WEIGHT_OBSERVATION as f64) as f32;
     let mono_score = record.monologue_trigger_count as f32 * ENGAGEMENT_WEIGHT_MONOLOGUE;
     conv_score + obs_score + mono_score
 }
@@ -462,7 +465,8 @@ fn compute_engagement_score(record: &EngagementRecord) -> f32 {
 /// 2. Co-presence query: find NPCs near any recorded player position (MovementHistoryBuffer).
 /// 3. Engagement scoring: score each co-present NPC via EngagementRecord.
 /// 4+5. Triangle routing: find Simmering triangle containing highest-scoring co-present NPC.
-///      Fallback (unentangled NPC, D-025): highest-tension Simmering triangle.
+///      v0.1: NPCs not assigned to any triangle are excluded (D-025 reference link routing deferred to v0.3+).
+///      Holds (no activation) if no triangle-assigned NPC is co-present.
 /// 6. Activation event: emit TriangleActivatedEvent, record in ActivationState.
 #[allow(clippy::too_many_arguments)]
 pub fn activation_pass(
@@ -484,15 +488,22 @@ pub fn activation_pass(
     if !activation_state.can_activate(time.tick) {
         return;
     }
-    // Gate 3: poll cadence — only run every ACTIVATION_CADENCE_TICKS
-    if time.tick % ACTIVATION_CADENCE_TICKS != 0 {
+    // Gate 3: poll cadence — only run every ACTIVATION_CADENCE_TICKS.
+    // Guard: at tick 0 all engagement scores are 0.0 and selection is random
+    // (e.g. contamination pre-set in a save file). Skip tick 0.
+    if time.tick == 0 || time.tick % ACTIVATION_CADENCE_TICKS != 0 {
         return;
     }
 
     // Step 2: co-presence query
     let npc_positions: Vec<(Entity, TilePosition)> =
         npcs.iter().map(|(e, pos, _)| (e, *pos)).collect();
-    let copresent = history.npcs_copresent_in_window(npc_positions.into_iter(), COPRESENCE_THRESHOLD);
+    let mut copresent = history.npcs_copresent_in_window(npc_positions.into_iter(), COPRESENCE_THRESHOLD);
+    // Deduplicate: the co-presence query can return the same entity more than once
+    // if multiple player positions fall near the same NPC. Without dedup, the NPC
+    // gets added to candidates twice, doubling its RNG weight.
+    copresent.sort_unstable();
+    copresent.dedup();
 
     // Snapshot simmering triangles for read (avoids double-borrow during activation write)
     let simmering: Vec<(Entity, TriangleState)> = triangles
@@ -553,12 +564,21 @@ pub fn activation_pass(
     };
     let (anchor_entity, tri_entity, anchor_score) = candidates[selected_idx];
 
-    // Retrieve triangle_id for the event
-    let triangle_id = simmering
+    // Retrieve triangle_id for the event.
+    // Guard: the entity was in the simmering snapshot we collected above, but in
+    // theory it could have been despawned between snapshot and lookup (unlikely but
+    // a panic in the gameplay loop is unacceptable).
+    let Some(triangle_id) = simmering
         .iter()
         .find(|(e, _)| *e == tri_entity)
         .map(|(_, s)| s.triangle_id)
-        .expect("target triangle must be in simmering snapshot");
+    else {
+        tracing::error!(
+            "activation_pass: target triangle entity {:?} missing from simmering snapshot — skipping activation at tick {}",
+            tri_entity, time.tick
+        );
+        return;
+    };
 
     // Step 6: set triangle phase to Active
     if let Ok((_, mut tri_state)) = triangles.get_mut(tri_entity) {