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/simulation/mod.rs

@@ -23,7 +23,6 @@ impl Plugin for SimulationPlugin {
         app.init_resource::<time::SimulationTime>()
             .insert_resource(rng::SimRng::new(0))
             .init_resource::<input::InputQueue>()
-            .init_resource::<interaction::NearbyInteractionBuffer>()
             .init_resource::<crate::knowledge::EntityRegistry>()
             .add_systems(
                 Update,