feat(simulation): complete interaction and monologue pipelines (#413, #414, #415)

Three fixes to make the gameplay loop functional end-to-end:

- Add Interactable component to NPC spawn so E-prompt detection works
- Build monologue trigger system (enter_location + time_idle) with
  MonologueBuffer/MonologueState components, wire through ObserverSnapshot
  as current_monologue field, decode on client and display via HUD
- Change PlayerAction::Interact from unit to struct variant carrying
  optional target_entity_id and verb fields

Bumps protocol version from 4 to 5. Regenerates MessagePack fixtures.
All 200 tests pass (170 unit + 30 integration).

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

server/src/perception/observation.rs

@@ -119,6 +119,7 @@ mod tests {
                 Facing(FacingDirection::North),
                 KnowledgeGraph::new(),
                 crate::simulation::interaction::NearbyInteractionBuffer::default(),
+                crate::simulation::monologue::MonologueBuffer::default(),
             ))
             .id();
         registry.register(player);
@@ -167,6 +168,7 @@ mod tests {
                 Facing(FacingDirection::North),
                 kg,
                 crate::simulation::interaction::NearbyInteractionBuffer::default(),
+                crate::simulation::monologue::MonologueBuffer::default(),
             ))
             .id();
         registry.register(player);
@@ -208,6 +210,7 @@ mod tests {
                 Facing::default(),
                 KnowledgeGraph::new(),
                 crate::simulation::interaction::NearbyInteractionBuffer::default(),
+                crate::simulation::monologue::MonologueBuffer::default(),
             ))
             .id();
         registry.register(player);