feat(@projects/@magic-civilization): 🌋 p3-26 gap 2 — volcanic eruption category

Third (and flagship) natural-event category, ported from GDScript process_volcanic:
apply_volcanic — center → volcano (quality 1), scorch disk turns non-water tiles to
scorched_terrain (desert, drier, quality 1), sulfate_aerosol injected in aerosol_radius
(climate physics converts to cooling next tick). dispatch_volcanic resolves the tier
config + picks a non-water center. Added to process_events dispatch. (Anchor/resource
spawns are magic → Game-3 deferred.)

Test: apply_volcanic_erupts_scorches_and_injects_aerosol (volcano center, desert scorch,
water spared, aerosol injected). mc-climate events 9/9. 3/12 categories live
(wildfire, drought, volcanic).

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

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

@@ -205,7 +205,8 @@ pub fn process_events(
         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.
+            "volcanic" => dispatch_volcanic(grid, cfg, tier, turn_seed, channel),
+            // TODO(p3-26 gap 2): impact/seismic/tsunami/plague/pandemic/marine/solar/glacial.
             _ => None,
         };
         if let Some(ev) = ev {
@@ -336,6 +337,82 @@ fn dispatch_wildfire(
     })
 }
 
+/// Apply a volcanic eruption: the center becomes a `volcano` (quality 1), a scorch disk
+/// turns non-water tiles to `scorched_terrain` (quality 1, drier), and sulfate aerosol is
+/// injected in `aerosol_radius` (the climate physics turns this into cooling next tick).
+/// Returns tiles affected. Mirrors GDScript `process_volcanic` (anchor/resource spawns are
+/// magic → Game-3 deferred).
+pub fn apply_volcanic(
+    grid: &mut mc_core::grid::GridState,
+    center: (i32, i32),
+    radius: i32,
+    scorched_terrain: &str,
+    moisture_loss: f32,
+    aerosol_strength: f32,
+    aerosol_radius: i32,
+) -> i32 {
+    use mc_core::algorithms::hex::{axial_to_offset, hex_spiral, offset_to_axial};
+    use mc_core::grid::biome_registry::{has_tag, BiomeTag};
+    if let Some(t) = grid.tile_mut(center.0, center.1) {
+        t.biome_label_id = "volcano".to_string();
+        t.quality = 1;
+    }
+    let (cq, cr) = offset_to_axial(center.0, center.1);
+    let mut affected = 1;
+    for (q, r) in hex_spiral(cq, cr, radius) {
+        let (col, row) = axial_to_offset(q, r);
+        if (col, row) == center {
+            continue;
+        }
+        if let Some(t) = grid.tile_mut(col, row) {
+            if !has_tag(&t.biome_label_id, BiomeTag::IsWater) {
+                t.biome_label_id = scorched_terrain.to_string();
+                t.moisture = (t.moisture - moisture_loss).max(0.0);
+                t.quality = 1;
+                affected += 1;
+            }
+        }
+    }
+    if aerosol_strength > 0.0 && aerosol_radius > 0 {
+        for (q, r) in hex_spiral(cq, cr, aerosol_radius) {
+            let (col, row) = axial_to_offset(q, r);
+            if let Some(t) = grid.tile_mut(col, row) {
+                t.sulfate_aerosol += aerosol_strength;
+            }
+        }
+    }
+    affected
+}
+
+/// Resolve the volcanic tier config + pick a non-water center, then erupt.
+fn dispatch_volcanic(
+    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 tc = cfg.raw.get("tiers").and_then(|t| t.get(tier.to_string()))?;
+    let radius = tc.get("radius").and_then(|v| v.as_i64()).unwrap_or(1) as i32;
+    let scorched = tc
+        .get("scorched_terrain")
+        .and_then(|v| v.as_str())
+        .unwrap_or("desert")
+        .to_string();
+    let moisture_loss = tc.get("scorched_moisture_loss").and_then(|v| v.as_f64()).unwrap_or(0.05) as f32;
+    let aero_str = tc.get("aerosol_strength").and_then(|v| v.as_f64()).unwrap_or(0.0) as f32;
+    let aero_rad = tc.get("aerosol_radius").and_then(|v| v.as_i64()).unwrap_or(0) as i32;
+    let center = pick_matching_tile(grid, channel, turn_seed, |b| !has_tag(b, BiomeTag::IsWater))?;
+    let affected = apply_volcanic(grid, center, radius, &scorched, moisture_loss, aero_str, aero_rad);
+    Some(FiredEvent {
+        category: "volcanic".to_string(),
+        tier,
+        center,
+        affected,
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -451,6 +528,31 @@ mod tests {
         assert_eq!(fired, fired2, "dispatch must be deterministic");
     }
 
+    #[test]
+    fn apply_volcanic_erupts_scorches_and_injects_aerosol() {
+        use mc_core::grid::GridState;
+        let mut grid = GridState::new(12, 12);
+        for t in &mut grid.tiles {
+            t.biome_label_id = "grassland".into();
+            t.moisture = 0.5;
+            t.quality = 3;
+            t.sulfate_aerosol = 0.0;
+        }
+        // ocean tile in scorch radius stays water.
+        if let Some(t) = grid.tile_mut(6, 5) {
+            t.biome_label_id = "ocean".into();
+        }
+        let affected = apply_volcanic(&mut grid, (5, 5), 2, "desert", 0.1, 0.2, 4);
+        assert!(affected >= 1);
+        assert_eq!(grid.tile(5, 5).unwrap().biome_label_id, "volcano", "center → volcano");
+        assert_eq!(grid.tile(5, 5).unwrap().quality, 1);
+        // a land tile in radius scorched to desert
+        assert_eq!(grid.tile(4, 5).unwrap().biome_label_id, "desert", "scorched → desert");
+        assert_eq!(grid.tile(6, 5).unwrap().biome_label_id, "ocean", "water not scorched");
+        // aerosol injected near center
+        assert!(grid.tile(5, 5).unwrap().sulfate_aerosol >= 0.2, "aerosol injected");
+    }
+
     #[test]
     fn apply_drought_dries_land_skips_water() {
         use mc_core::grid::GridState;