feat(simulation): add tick rate scaling system

Replace binary paused flag with TickRate enum (Full/Half/Paused)
per D-052. Full rate advances every frame, Half every 2 frames
via fractional accumulation, Paused blocks all advances. Add
SetTickRate player action for client-driven rate changes.
Implements ticket #406.

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

server/src/simulation/input.rs

@@ -5,7 +5,7 @@
 use crate::bridge::types::{PlayerAction, PlayerInput};
 use crate::perception::vision_cone::{facing_from_delta, Facing};
 use crate::simulation::movement::{MoveIntent, PlayerCharacter, TilePosition};
-use crate::simulation::time::SimulationTime;
+use crate::simulation::time::{SimulationTime, TickRate};
 use bevy_ecs::prelude::*;
 use std::collections::VecDeque;
 
@@ -75,13 +75,17 @@ pub fn process_player_input(
             PlayerAction::MoveSoutheast => apply_move(&player_query, &mut commands, 1, 1),
             PlayerAction::MoveSouthwest => apply_move(&player_query, &mut commands, -1, 1),
             PlayerAction::Pause => {
-                time.paused = true;
+                time.tick_rate = TickRate::Paused;
                 tracing::debug!("Simulation paused by player input");
             }
             PlayerAction::Unpause => {
-                time.paused = false;
+                time.tick_rate = TickRate::Full;
                 tracing::debug!("Simulation unpaused by player input");
             }
+            PlayerAction::SetTickRate(rate) => {
+                time.tick_rate = rate;
+                tracing::debug!("Tick rate set to {:?} by player input", rate);
+            }
             PlayerAction::Interact => {
                 tracing::trace!("Interact action — no-op for Sprint 1");
             }
@@ -160,10 +164,7 @@ mod tests {
     fn process_input_move_creates_intent() {
         let mut world = bevy_ecs::world::World::new();
         world.insert_resource(InputQueue::default());
-        world.insert_resource(SimulationTime {
-            tick: 0,
-            paused: false,
-        });
+        world.insert_resource(SimulationTime::default());
 
         let player = world
             .spawn((PlayerCharacter, TilePosition::new(5, 5, 0)))
@@ -183,13 +184,10 @@ mod tests {
     }
 
     #[test]
-    fn process_input_pause_toggles() {
+    fn process_input_pause_sets_paused() {
         let mut world = bevy_ecs::world::World::new();
         world.insert_resource(InputQueue::default());
-        world.insert_resource(SimulationTime {
-            tick: 0,
-            paused: false,
-        });
+        world.insert_resource(SimulationTime::default());
 
         world.resource_mut::<InputQueue>().push(PlayerInput {
             tick: 0,
@@ -200,17 +198,32 @@ mod tests {
         schedule.add_systems(process_player_input);
         schedule.run(&mut world);
 
-        assert!(world.resource::<SimulationTime>().paused);
+        assert_eq!(world.resource::<SimulationTime>().tick_rate, TickRate::Paused);
+    }
+
+    #[test]
+    fn process_input_set_tick_rate() {
+        let mut world = bevy_ecs::world::World::new();
+        world.insert_resource(InputQueue::default());
+        world.insert_resource(SimulationTime::default());
+
+        world.resource_mut::<InputQueue>().push(PlayerInput {
+            tick: 0,
+            action: PlayerAction::SetTickRate(TickRate::Half),
+        });
+
+        let mut schedule = bevy_ecs::schedule::Schedule::default();
+        schedule.add_systems(process_player_input);
+        schedule.run(&mut world);
+
+        assert_eq!(world.resource::<SimulationTime>().tick_rate, TickRate::Half);
     }
 
     #[test]
     fn process_input_no_player_no_panic() {
         let mut world = bevy_ecs::world::World::new();
         world.insert_resource(InputQueue::default());
-        world.insert_resource(SimulationTime {
-            tick: 0,
-            paused: false,
-        });
+        world.insert_resource(SimulationTime::default());
 
         world.resource_mut::<InputQueue>().push(PlayerInput {
             tick: 0,
@@ -227,10 +240,7 @@ mod tests {
     fn process_input_future_tick_ignored() {
         let mut world = bevy_ecs::world::World::new();
         world.insert_resource(InputQueue::default());
-        world.insert_resource(SimulationTime {
-            tick: 0,
-            paused: false,
-        });
+        world.insert_resource(SimulationTime::default());
 
         let player = world
             .spawn((PlayerCharacter, TilePosition::new(5, 5, 0)))