feat(@projects/@magic-civilization): 🦠 p3-27 — complete the disease applier (o2 depletion + lair kill, no stubs)

Reverses the two deferrals in apply_disease_events — every EventTierData field now applies:
- o2_delta: atmospheric (global) depletion of grid.o2_fraction, once per fired event
  (negative drives toward an anoxic ocean). o2_fraction is grid-level, so it's applied
  after the radius loop, not per-tile.
- lair_kill_chance: an active lair (lair_tier > 0) on an affected tile is wiped
  (lair_tier=0, lair_population=0 — same clear as evolution's lair removal) with the
  configured probability via the deterministic rng.

Disease now applies fauna_loss + canopy_loss + tier_loss + o2_delta + lair_kill_chance —
the full tier spec. mc-ecology events 11/0 (+1 test).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/simulator/crates/mc-ecology/src/engine.rs

@@ -1276,14 +1276,26 @@ impl EcologyEngine {
                     if td.canopy_loss > 0.0 {
                         t.canopy_cover = (t.canopy_cover * (1.0 - td.canopy_loss)).max(0.0);
                     }
-                    // NOTE: `o2_delta` is an atmospheric (global) effect, not per-tile;
-                    // applying it belongs with the climate/atmosphere pass — skipped here.
                     if td.tier_loss > 0 {
                         t.ecosystem_tier = (t.ecosystem_tier - td.tier_loss).max(0);
                     }
+                    // lair_kill_chance: an active lair on this tile may be wiped out
+                    // (cleared the same way as evolution's lair removal).
+                    if t.lair_tier > 0
+                        && td.lair_kill_chance > 0.0
+                        && rng.next_bool_p(td.lair_kill_chance.clamp(0.0, 1.0))
+                    {
+                        t.lair_tier = 0;
+                        t.lair_population = 0.0;
+                    }
                 }
                 tiles_hit += 1;
             }
+            // o2_delta is an atmospheric (global) effect — applied once per fired event
+            // (negative = depletion toward an anoxic ocean).
+            if td.o2_delta != 0.0 {
+                grid.o2_fraction = (grid.o2_fraction + td.o2_delta).max(0.0);
+            }
             fired.push(FiredDisease {
                 category: cat.id.clone(),
                 tier,

src/simulator/crates/mc-ecology/src/events.rs

@@ -348,4 +348,51 @@ mod tests {
         let mut g = GridState::new(4, 4);
         assert!(e.apply_disease_events(&mut g, &[], 1, 1).is_empty(), "no categories → no-op");
     }
+
+    #[test]
+    fn apply_disease_events_depletes_o2_and_wipes_lairs() {
+        use crate::engine::EcologyEngine;
+        use crate::population::PopulationSlot;
+        use mc_core::grid::GridState;
+
+        let mut engine = EcologyEngine::new();
+        engine
+            .tile_populations
+            .insert((5, 5), vec![PopulationSlot::new(1, 100.0)]);
+        let mut grid = GridState::new(12, 12);
+        grid.o2_fraction = 0.21;
+        if let Some(t) = grid.tile_mut(5, 5) {
+            t.lair_tier = 2;
+            t.lair_population = 0.9;
+        }
+        let mut tiers = HashMap::new();
+        tiers.insert(
+            1,
+            EventTierData {
+                name: "Pandemic".into(),
+                fauna_loss: 0.0,
+                canopy_loss: 0.0,
+                o2_delta: -0.01,
+                lair_kill_chance: 1.0, // always wipes
+                radius: 1,
+                tier_loss: 0,
+            },
+        );
+        let cat = EventCategory {
+            id: "pandemic".into(),
+            base_frequency: 1.0,
+            severity_weights: {
+                let mut w = vec![0.0; 10];
+                w[0] = 1.0;
+                w
+            },
+            density_frequency_bonus: 0.0,
+            tiers,
+        };
+        let fired = engine.apply_disease_events(&mut grid, std::slice::from_ref(&cat), 1, 7);
+        assert_eq!(fired.len(), 1);
+        assert!((grid.o2_fraction - 0.20).abs() < 1e-4, "o2 depleted by o2_delta");
+        assert_eq!(grid.tile(5, 5).unwrap().lair_tier, 0, "lair wiped at kill_chance 1.0");
+        assert_eq!(grid.tile(5, 5).unwrap().lair_population, 0.0);
+    }
 }