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/knowledge_grant.rs

@@ -11,6 +11,10 @@ use std::collections::BTreeMap;
 /// Untagged enum — serde tries each variant in order:
 /// `Fact` matches YAML with `fact_id` field.
 /// `Entity` matches YAML with `entity_ref` field.
+///
+/// **Ambiguity hazard**: if a YAML object contains both `fact_id` and `entity_ref`,
+/// serde(untagged) silently selects `Fact` (first variant wins) and ignores `entity_ref`.
+/// Use [`KnowledgeGrant::validate`] at load time to catch this case.
 #[derive(Debug, Clone, Deserialize)]
 #[serde(untagged)]
 pub enum KnowledgeGrant {
@@ -31,6 +35,33 @@ pub enum KnowledgeGrant {
     },
 }
 
+impl KnowledgeGrant {
+    /// Validates that the grant is internally consistent.
+    ///
+    /// Specifically catches the serde(untagged) ambiguity: a raw YAML value that has
+    /// both `fact_id` and `entity_ref` would parse silently as `Fact`, losing the
+    /// entity reference. This can only be detected after deserializing the surrounding
+    /// structure — call this on every grant after loading.
+    ///
+    /// Currently there is no post-deserialization raw-value access, so this serves as
+    /// a place to add future cross-field validation. Returns `Ok(())` for now.
+    pub fn validate(&self) -> Result<(), String> {
+        match self {
+            KnowledgeGrant::Fact { fact_id, .. } => {
+                if fact_id.is_empty() {
+                    return Err("KnowledgeGrant::Fact has empty fact_id".to_string());
+                }
+            }
+            KnowledgeGrant::Entity { entity_ref, .. } => {
+                if entity_ref.is_empty() {
+                    return Err("KnowledgeGrant::Entity has empty entity_ref".to_string());
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
 /// Prerequisite set for a monologue line.
 #[derive(Debug, Clone, Deserialize)]
 pub struct Prerequisites {