feat(simulation): add tile collision system (#236)

TilePosition component with discrete grid coordinates, flat-storage
WalkabilityMap resource with O(1) can_move_to() lookup, MoveIntent
component and validate_movement system. Movement validated against
walkability map each tick, blocking all NPC and player movement
through unwalkable tiles. 11 unit tests + 1 integration test.

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

server/src/simulation/mod.rs

@@ -2,8 +2,10 @@
 // Implements deterministic tick-based simulation (D-010 principle 4)
 
 use bevy_app::prelude::*;
+use bevy_ecs::schedule::IntoScheduleConfigs;
 
 pub mod input;
+pub mod movement;
 pub mod rng;
 pub mod tier;
 pub mod time;
@@ -18,7 +20,11 @@ impl Plugin for SimulationPlugin {
         app.init_resource::<time::SimulationTime>()
             .insert_resource(rng::SimRng::new(0))
             .init_resource::<input::InputQueue>()
-            .add_systems(Update, time::advance_tick);
+            .add_systems(Update, time::advance_tick)
+            .add_systems(
+                Update,
+                movement::validate_movement.after(time::advance_tick),
+            );
 
         tracing::debug!("SimulationPlugin initialized");
     }