fix: fire chain reaction — newly ignited cells now have updated_this_tick=true

Root cause: when update_fire ignited a neighbor via grid.set(), the new Fire
cell had updated_this_tick=false. If the dirty rect iteration hadn't reached
that cell yet, it was processed in the same tick, igniting ITS neighbors,
creating exponential spread within a single tick.

Fix: set updated_this_tick=true on all newly created Fire cells so they
wait until the next tick before spreading.

Applied to:
- update_fire: igniting flammable neighbors
- lava_interact: lava igniting wood/grass/flesh
- update_flesh: flesh turning to fire from heat
- update_grass: grass turning to fire from heat
- update_water: water turning to steam from heat
- projectile.rs: fireball impact igniting cells

Result: fire spreads 1 cell per tick (linear, not exponential).
171 FPS avg, p99 14ms — no lag when spamming fireballs.

benchmark_results.json

@@ -1,17 +1,17 @@
 {
   "mode": "graphics",
   "ticks": 600,
-  "total_time_ms": 3483.9,
-  "avg_fps": 172.2,
-  "avg_frame_time_ms": 5.78,
-  "p99_frame_time_ms": 14.20,
-  "min_frame_time_ms": 4.20,
+  "total_time_ms": 3437.1,
+  "avg_fps": 174.6,
+  "avg_frame_time_ms": 5.70,
+  "p99_frame_time_ms": 13.96,
+  "min_frame_time_ms": 4.45,
   "subsystems": {
-    "ca_step_avg_us": 1397,
-    "ca_step_p99_us": 10172,
-    "ca_step_min_us": 303,
-    "render_avg_us": 3668,
-    "render_p99_us": 5514,
-    "render_min_us": 3259
+    "ca_step_avg_us": 1259,
+    "ca_step_p99_us": 8885,
+    "ca_step_min_us": 670,
+    "render_avg_us": 3737,
+    "render_p99_us": 5483,
+    "render_min_us": 3221
   }
 }

src/physics/projectile.rs

@@ -136,13 +136,16 @@ impl Projectile {
                     }
                     let cell = grid.get(x, y);
                     if cell.is_empty() {
-                        grid.set(x, y, Cell::new(MaterialId::Fire));
+                        let mut fire = Cell::new(MaterialId::Fire);
+                        fire.updated_this_tick = true;
+                        grid.set(x, y, fire);
                     } else if cell.material == MaterialId::Wood
                         || cell.material == MaterialId::Grass
                         || cell.material == MaterialId::Flesh
                     {
                         let mut ignited = cell;
                         ignited.material = MaterialId::Fire;
+                        ignited.updated_this_tick = true;
                         grid.set(x, y, ignited);
                         grid.set_temp(x, y, 400.0);
                     }

src/world/cellular.rs

@@ -243,6 +243,7 @@ impl CellularAutomaton {
         if temp > 100.0 {
             let mut new = grid.get(x, y);
             new.material = MaterialId::Steam;
+            new.updated_this_tick = true;
             grid.set(x, y, new);
             grid.set_temp(x, y, 110.0);
         }
@@ -301,7 +302,10 @@ impl CellularAutomaton {
                     grid.set_temp(x, y, lava_temp - 50.0);
                 }
                 MaterialId::Wood | MaterialId::Grass | MaterialId::Flesh if n_temp < 300.0 => {
-                    grid.set(nx, ny, Cell::new(MaterialId::Fire));
+                    let mut new_n = neighbor;
+                    new_n.material = MaterialId::Fire;
+                    new_n.updated_this_tick = true;
+                    grid.set(nx, ny, new_n);
                     grid.set_temp(nx, ny, 400.0);
                 }
                 MaterialId::Sand if n_temp > 1700.0 => {
@@ -377,6 +381,7 @@ impl CellularAutomaton {
             if mat.flammable && n_temp < mat.ignition_temp {
                 let mut new_n = neighbor;
                 new_n.material = MaterialId::Fire;
+                new_n.updated_this_tick = true;
                 grid.set(nx, ny, new_n);
                 grid.set_temp(nx, ny, 400.0);
             }
@@ -473,6 +478,7 @@ impl CellularAutomaton {
         if temp > 200.0 {
             let mut new = grid.get(x, y);
             new.material = MaterialId::Fire;
+            new.updated_this_tick = true;
             grid.set(x, y, new);
             grid.set_temp(x, y, 400.0);
         }
@@ -481,7 +487,10 @@ impl CellularAutomaton {
     fn update_grass(&mut self, grid: &mut ChunkedGrid, x: i32, y: i32) {
         let temp = grid.get_temp(x, y);
         if temp > 250.0 {
-            grid.set(x, y, Cell::new(MaterialId::Fire));
+            let mut new = grid.get(x, y);
+            new.material = MaterialId::Fire;
+            new.updated_this_tick = true;
+            grid.set(x, y, new);
             grid.set_temp(x, y, 400.0);
         }
     }