feat(simulation): Sprint 19 — 7 server systems

Protocol handshake (#555): HandshakeMessage as first IPC frame,
HandshakeState resource, forward-compatible input handling.

State serialization (#96): serialize_npc_to_frozen/deserialize with
full D-024 axis coverage (10 new optional fields on NpcSaveState).

Scope tags (#98): ScopeTagKind enum, ScopePinned marker, automatic
assignment from KnowledgeGraph and RelationshipGraph.

Timestamp eviction (#97): LastInteractionTick, SimSpacePressure,
BinaryHeap LRU eviction respecting ScopePinned entities.

Save/load (#553): save_to_file/load_from_file via MessagePack,
SaveGame/LoadGame IPC commands, SaveLoadResultWire on snapshot.

Test infrastructure (#200): Layer 3 integration test entry point,
three-layer architecture documented per D-030.

Information boundary tests (#272): 4 negative tests proving no
passive KG leakage, LOS fog holds, tier boundary holds, save
isolation per NPC.

1063 tests passing, 0 failures.

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

server/src/simulation/mod.rs

@@ -22,6 +22,7 @@ pub mod poi;
 pub mod poi_discovery;
 pub mod pressure;
 pub mod rng;
+pub mod save_io;
 pub mod save_state;
 pub mod sound;
 pub mod spatial;
@@ -43,6 +44,7 @@ impl Plugin for SimulationPlugin {
         app.init_resource::<time::SimulationTime>()
             .insert_resource(rng::SimRng::new(0))
             .init_resource::<input::InputQueue>()
+            .init_resource::<save_io::SaveLoadPending>()
             .init_resource::<crate::knowledge::EntityRegistry>()
             .init_resource::<sound::SoundEventQueue>()
             .init_resource::<spatial::NaiveSpatialIndex>()
@@ -61,6 +63,12 @@ impl Plugin for SimulationPlugin {
                 Update,
                 (
                     input::process_player_input,
+                    // execute_save_load is an exclusive system (takes &mut World).
+                    // Must run after process_player_input (which queues the command)
+                    // and before compute_observer_snapshot (which consumes the result).
+                    save_io::execute_save_load
+                        .after(input::process_player_input)
+                        .before(crate::perception::observer::compute_observer_snapshot),
                     pathfinding::compute_paths.after(input::process_player_input),
                     path_follow::follow_paths.after(pathfinding::compute_paths),
                     movement::validate_movement.after(path_follow::follow_paths),