fix: ssd streaming and deepseek fixes

src/engine.rs

@@ -376,6 +376,14 @@ struct ResidentSlot {
     last_store_tokens: u32,
 }
 
+#[cfg(target_os = "macos")]
+impl Drop for Generator {
+    fn drop(&mut self) {
+        // Match ds4: every resident session graph must die before Metal cleanup.
+        self.resident_sessions.clear();
+    }
+}
+
 #[cfg(target_os = "macos")]
 struct SimulatedMemory {
     address: std::ptr::NonNull<std::ffi::c_void>,
@@ -524,9 +532,10 @@ impl Generator {
             resident_active: None,
             resident_limit: std::env::var("DS4_RESIDENT_SESSIONS")
                 .ok()
-                .and_then(|value| value.parse().ok())
+                .and_then(|value| value.parse::<usize>().ok())
                 .filter(|limit| *limit > 0)
-                .unwrap_or(4),
+                .unwrap_or(1)
+                .saturating_sub(1),
         })
     }
 
@@ -908,21 +917,23 @@ impl Generator {
             .map_or((None, 0), |slot| (Some(slot.state), slot.last_store_tokens));
         self.executor.swap_resident_state(&mut incoming_state)?;
         if let (Some(previous), Some(outgoing)) = (self.resident_active.take(), incoming_state) {
-            // ponytail: four resident sessions bound memory; raise
-            // DS4_RESIDENT_SESSIONS when the machine can hold more KV state.
-            if self.resident_sessions.len() >= self.resident_limit {
-                let evicted = self.resident_sessions.keys().next().cloned();
-                if let Some(evicted) = evicted {
-                    self.resident_sessions.remove(&evicted);
+            // DS4 defaults to one active session; opt into its batched-server
+            // behavior with DS4_RESIDENT_SESSIONS when memory permits.
+            if self.resident_limit != 0 {
+                if self.resident_sessions.len() >= self.resident_limit {
+                    let evicted = self.resident_sessions.keys().next().cloned();
+                    if let Some(evicted) = evicted {
+                        self.resident_sessions.remove(&evicted);
+                    }
                 }
+                self.resident_sessions.insert(
+                    previous,
+                    ResidentSlot {
+                        state: outgoing,
+                        last_store_tokens: self.last_store_tokens,
+                    },
+                );
             }
-            self.resident_sessions.insert(
-                previous,
-                ResidentSlot {
-                    state: outgoing,
-                    last_store_tokens: self.last_store_tokens,
-                },
-            );
         }
         self.resident_active = Some(key);
         self.checkpoint = None;