refactor(server): NearbyInteractionBuffer from Resource to Component

Per-entity Component attached to PlayerCharacter instead of global
Resource. Makes the interaction buffer multiplayer-ready (D-009) —
each observer gets their own buffer without pipeline refactoring.

Updated all 8 files touching the buffer: system signatures, player
spawn bundles, and ~30 test spawn sites.

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

server/src/perception/observation.rs

@@ -101,7 +101,6 @@ mod tests {
         world.init_resource::<SnapshotBuffer>();
         world.init_resource::<KnowledgeEventQueue>();
         world.init_resource::<EntityRegistry>();
-        world.init_resource::<crate::simulation::interaction::NearbyInteractionBuffer>();
         world
     }
 
@@ -116,6 +115,7 @@ mod tests {
                 TilePosition::new(16, 16, 0),
                 Facing(FacingDirection::North),
                 KnowledgeGraph::new(),
+                crate::simulation::interaction::NearbyInteractionBuffer::default(),
             ))
             .id();
         registry.register(player);
@@ -159,6 +159,7 @@ mod tests {
                 TilePosition::new(16, 16, 0),
                 Facing(FacingDirection::North),
                 kg,
+                crate::simulation::interaction::NearbyInteractionBuffer::default(),
             ))
             .id();
         registry.register(player);
@@ -195,6 +196,7 @@ mod tests {
                 TilePosition::new(16, 16, 0),
                 Facing::default(),
                 KnowledgeGraph::new(),
+                crate::simulation::interaction::NearbyInteractionBuffer::default(),
             ))
             .id();
         registry.register(player);