Exorcise field ghosts: remove_region on free, dead allocations leave the field

63,000 cold regions in the Lenia field were freed allocations that never
got removed. The field was paying to cool corpses — stepping temperature
on memory that no longer exists.

Fix: add LeniaField::remove_region(), wire into Pipeline::process_free().
Freed allocations are removed from the field, their energy reclaimed,
and their neighbor links cleaned up. The field only contains live memory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

rust_core/src/lenia.rs

@@ -184,6 +184,24 @@ impl LeniaField {
         self.regions.insert(id, region);
     }
 
+    /// Remove a region from the field — called when an allocation is freed.
+    /// Reclaims the energy and removes from all tracking structures.
+    /// Dead allocations must not haunt the field.
+    pub fn remove_region(&mut self, id: u32) {
+        if let Some(region) = self.regions.remove(&id) {
+            let energy = region.temperature * (region.size_bytes as f64 / (1024.0 * 1024.0));
+            self.total_energy -= energy;
+            if self.total_energy < 0.0 {
+                self.total_energy = 0.0;
+            }
+        }
+        self.neighbors.remove(&id);
+        // Also remove this id from other regions' neighbor lists
+        for (_rid, nbrs) in self.neighbors.iter_mut() {
+            nbrs.retain(|(nid, _)| *nid != id);
+        }
+    }
+
     /// Set neighborhood connections from graph edges
     pub fn set_neighbors(&mut self, id: u32, neighbors: Vec<(u32, f64)>) {
         self.neighbors.insert(id, neighbors);

rust_core/src/pipeline.rs

@@ -340,7 +340,10 @@ impl Pipeline {
     pub fn process_free(&mut self, address: usize) {
         self.condenser.unregister(address);
         self.address_to_path.remove(&address);
-        self.address_to_field_id.remove(&address);
+        // Remove from Lenia field — dead allocations don't get thermal cycles
+        if let Some(field_id) = self.address_to_field_id.remove(&address) {
+            self.field.remove_region(field_id);
+        }
     }
 
     /// Rebuild the graph from accumulated events and retrain the predictor.