fix(simulation): address PR review items for #655 content removal

- line_pool.rs: add trace! when active_situations is empty (Layer 2
  will silently filter all lines — important for content debugging)
- knowledge_grant.rs: document serde(untagged) ambiguity hazard; add
  KnowledgeGrant::validate() for load-time field validation
- monologue.rs: annotate trigger_monologue as schedule ordering anchor;
  fix stale "periodic trigger cooldown tracking" doc reference in
  trigger_event_monologue (COOLDOWN_TICKS removed in #655)

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

server/src/simulation/line_pool.rs

@@ -13,6 +13,7 @@ use std::fmt;
 use std::str::FromStr;
 
 use bevy_ecs::prelude::*;
+use tracing::trace;
 
 use crate::simulation::knowledge_grant::{KnowledgeGrant, Prerequisites};
 
@@ -363,6 +364,14 @@ impl LinePoolIndex {
         active_situations: &[Situation],
         player_trust: TrustTier,
     ) -> Vec<&IndexedDialogueLine> {
+        if active_situations.is_empty() {
+            trace!(
+                location,
+                role,
+                "query_dialogue: active_situations is empty — Layer 2 will filter all lines"
+            );
+        }
+
         let key = (location.to_string(), role.to_string());
         let Some(pool) = self.dialogue.get(&key) else {
             return Vec::new();