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/main.rs

@@ -14,6 +14,7 @@ use settled_reach_server::npc::{
     Want, WantKind,
 };
 use settled_reach_server::perception::vision_cone::Facing;
+use settled_reach_server::simulation::interaction::NearbyInteractionBuffer;
 use settled_reach_server::simulation::movement::{PlayerCharacter, TilePosition, WalkabilityMap};
 use settled_reach_server::simulation::path_follow::MovementSpeed;
 use settled_reach_server::simulation::time::DayPhase;
@@ -68,6 +69,7 @@ fn main() {
             TilePosition::new(16, 16, 0),
             Facing::default(),
             KnowledgeGraph::new(),
+            NearbyInteractionBuffer::default(),
         ))
         .id();
     registry.register(player);