feat(@projects/@magic-civilization): 🏜️ p3-26 gap 2 — drought category + shared tile-picker

Second natural-event category, ported from GDScript process_drought:
- apply_drought: reduce moisture (min 0) in a hex disk, skipping water tiles
  (mc_core::grid::biome_registry::has_tag(.., IsWater)). Returns tiles affected.
- dispatch_drought: resolve tier (radius/moisture_loss) + pick a non-water center.
- Extracted pick_matching_tile (shared deterministic tile selection); refactored
  dispatch_wildfire to use it. process_events now dispatches wildfire + drought.

Test: apply_drought_dries_land_skips_water (land moisture drops, ocean untouched).
mc-climate events 8/8. Remaining categories (volcanic/seismic/tsunami/plague/marine/…)
follow the same pattern.

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

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

@@ -202,16 +202,89 @@ pub fn process_events(
             continue;
         }
         let tier = roll_severity(&cfg.severity_weights, turn_seed, channel, max_tier);
-        if *category == "wildfire" {
-            if let Some(ev) = dispatch_wildfire(grid, cfg, tier, turn_seed, channel) {
-                fired.push(ev);
-            }
+        let ev = match *category {
+            "wildfire" => dispatch_wildfire(grid, cfg, tier, turn_seed, channel),
+            "drought" => dispatch_drought(grid, cfg, tier, turn_seed, channel),
+            // TODO(p3-26 gap 2): volcanic/seismic/tsunami/plague/marine/… handlers.
+            _ => None,
+        };
+        if let Some(ev) = ev {
+            fired.push(ev);
         }
-        // TODO(p3-26 gap 2): volcanic/seismic/drought/… effect handlers.
     }
     fired
 }
 
+/// Deterministically pick a tile whose biome satisfies `matches`, using the category's
+/// noise channel. Returns `None` if no tile matches. Internal-determinism only (the
+/// headless sim doesn't byte-match the live game's Godot RNG).
+fn pick_matching_tile<F: Fn(&str) -> bool>(
+    grid: &mc_core::grid::GridState,
+    channel: f64,
+    turn_seed: f64,
+    matches: F,
+) -> Option<(i32, i32)> {
+    let tiles: Vec<(i32, i32)> = (0..grid.height)
+        .flat_map(|row| (0..grid.width).map(move |col| (col, row)))
+        .filter(|&(c, r)| grid.tile(c, r).is_some_and(|t| matches(&t.biome_label_id)))
+        .collect();
+    if tiles.is_empty() {
+        return None;
+    }
+    let pick = (hash_noise(channel, 1.0, turn_seed) * tiles.len() as f64) as usize;
+    Some(tiles[pick.min(tiles.len() - 1)])
+}
+
+/// Apply a drought: reduce moisture (min 0) in a hex disk around `center`, skipping water
+/// tiles. Returns tiles affected. Mirrors GDScript `process_drought`.
+pub fn apply_drought(
+    grid: &mut mc_core::grid::GridState,
+    center: (i32, i32),
+    radius: i32,
+    moisture_loss: f32,
+) -> i32 {
+    use mc_core::algorithms::hex::{axial_to_offset, hex_spiral, offset_to_axial};
+    use mc_core::grid::biome_registry::{has_tag, BiomeTag};
+    let (cq, cr) = offset_to_axial(center.0, center.1);
+    let mut affected = 0;
+    for (q, r) in hex_spiral(cq, cr, radius) {
+        let (col, row) = axial_to_offset(q, r);
+        if let Some(t) = grid.tile_mut(col, row) {
+            if !has_tag(&t.biome_label_id, BiomeTag::IsWater) {
+                t.moisture = (t.moisture - moisture_loss).max(0.0);
+                affected += 1;
+            }
+        }
+    }
+    affected
+}
+
+/// Resolve the drought tier config + pick a non-water center, then dry the area.
+fn dispatch_drought(
+    grid: &mut mc_core::grid::GridState,
+    cfg: &EventCategoryConfig,
+    tier: usize,
+    turn_seed: f64,
+    channel: f64,
+) -> Option<FiredEvent> {
+    use mc_core::grid::biome_registry::{has_tag, BiomeTag};
+    let tier_cfg = cfg.raw.get("tiers").and_then(|t| t.get(tier.to_string()))?;
+    let radius = tier_cfg.get("radius").and_then(|v| v.as_i64()).unwrap_or(2) as i32;
+    let moisture_loss = tier_cfg.get("moisture_loss").and_then(|v| v.as_f64()).unwrap_or(0.05) as f32;
+    let center =
+        pick_matching_tile(grid, channel, turn_seed, |b| !has_tag(b, BiomeTag::IsWater))?;
+    let affected = apply_drought(grid, center, radius, moisture_loss);
+    if affected == 0 {
+        return None;
+    }
+    Some(FiredEvent {
+        category: "drought".to_string(),
+        tier,
+        center,
+        affected,
+    })
+}
+
 /// Resolve the wildfire tier config + pick a deterministic forest center, then burn.
 fn dispatch_wildfire(
     grid: &mut mc_core::grid::GridState,
@@ -240,18 +313,9 @@ fn dispatch_wildfire(
 
     // Deterministic forest-tile pick (headless sim needs internal determinism, not a
     // byte-match with the live game's Godot RNG).
-    let forests: Vec<(i32, i32)> = (0..grid.height)
-        .flat_map(|row| (0..grid.width).map(move |col| (col, row)))
-        .filter(|&(c, r)| {
-            grid.tile(c, r)
-                .is_some_and(|t| target_terrain.iter().any(|b| b.as_str() == t.biome_label_id))
-        })
-        .collect();
-    if forests.is_empty() {
-        return None;
-    }
-    let pick = (hash_noise(channel, 1.0, turn_seed) * forests.len() as f64) as usize;
-    let center = forests[pick.min(forests.len() - 1)];
+    let center = pick_matching_tile(grid, channel, turn_seed, |b| {
+        target_terrain.iter().any(|t| t.as_str() == b)
+    })?;
     let affected = apply_wildfire(
         grid,
         center,
@@ -387,6 +451,29 @@ mod tests {
         assert_eq!(fired, fired2, "dispatch must be deterministic");
     }
 
+    #[test]
+    fn apply_drought_dries_land_skips_water() {
+        use mc_core::grid::GridState;
+        let mut grid = GridState::new(10, 10);
+        for t in &mut grid.tiles {
+            t.biome_label_id = "grassland".into();
+            t.moisture = 0.6;
+        }
+        if let Some(t) = grid.tile_mut(5, 5) {
+            t.biome_label_id = "ocean".into();
+            t.moisture = 1.0;
+        }
+        let affected = apply_drought(&mut grid, (4, 5), 2, 0.2);
+        assert!(affected >= 1, "should dry some land");
+        let land = grid.tile(4, 5).unwrap();
+        assert!(
+            (land.moisture - 0.4).abs() < 1e-5,
+            "land dried 0.6-0.2=0.4 (got {})",
+            land.moisture
+        );
+        assert_eq!(grid.tile(5, 5).unwrap().moisture, 1.0, "water tile not dried");
+    }
+
     #[test]
     fn load_event_configs_parses_real_categories() {
         let dir = std::path::Path::new(env!("CARGO_MANIFEST_DIR"))