fix(simulation): PR #68 review — version bump, tracing warns, test coverage

- Bump PROTOCOL_VERSION 14 → 15 for save_result field addition
- Add tracing::warn on SaveLoadPending command overwrite (double-tap F5)
- Add tracing::warn on KnowledgeGraph::new() fallback during save
- Fix misleading WouldBlock comment in tcp.rs
- Document SimSpacePressure.active_count pre-eviction timing
- Document entity-based eviction tie-breaking non-determinism
- Add ScopePinned eviction survival regression test
- Regenerate msgpack fixtures for protocol v15

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

server/src/simulation/save_io.rs

@@ -84,9 +84,10 @@ pub fn save_to_file(path: &Path, world: &mut World) -> Result<(), SaveLoadError>
     // Player knowledge graph — the observer's epistemics at save time
     let player_knowledge = {
         let mut q = world.query_filtered::<&KnowledgeGraph, With<PlayerCharacter>>();
-        q.single(world)
-            .cloned()
-            .unwrap_or_else(|_| KnowledgeGraph::new())
+        q.single(world).cloned().unwrap_or_else(|_| {
+            tracing::warn!("save_to_file: no PlayerCharacter with KnowledgeGraph found — saving empty graph");
+            KnowledgeGraph::new()
+        })
     };
 
     // Global NPC social web
@@ -610,6 +611,32 @@ mod tests {
         assert!(result.error.is_some(), "error message should be present");
     }
 
+    // -----------------------------------------------------------------------
+    // Overwrite behaviour
+    // -----------------------------------------------------------------------
+
+    /// When two commands arrive in the same tick, the second overwrites the first.
+    /// The warn! in process_player_input fires; here we just confirm last-write-wins.
+    #[test]
+    fn pending_command_overwrite_last_write_wins() {
+        let mut pending = SaveLoadPending::default();
+
+        pending.pending = Some(SaveLoadCommand::Save {
+            path: PathBuf::from("/tmp/first.msgpack"),
+        });
+        // Overwrite with a Load command
+        pending.pending = Some(SaveLoadCommand::Load {
+            path: PathBuf::from("/tmp/second.msgpack"),
+        });
+
+        match pending.pending.unwrap() {
+            SaveLoadCommand::Load { ref path } => {
+                assert_eq!(path.to_str().unwrap(), "/tmp/second.msgpack");
+            }
+            other => panic!("expected Load, got {:?}", other),
+        }
+    }
+
     // -----------------------------------------------------------------------
     // SaveLoadError display
     // -----------------------------------------------------------------------