refactor: decompose tick_once into helper methods

Extract drain_spawns() (phases 1+4, dedup) and cleanup_dead_actors()
    (phase 7, 48 lines of complex death notification + GC logic).

    tick_once: 196 → 133 lines (−32%), max nesting depth: 5 → 3.

    Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski

Cargo.lock

@@ -2747,6 +2747,7 @@ dependencies = [
 name = "swactor-wasm-actor"
 version = "0.1.0"
 dependencies = [
+ "proptest",
  "swactor",
  "wasmtime",
  "wat",

crates/wasm-actor/Cargo.toml

@@ -10,3 +10,4 @@ wasmtime = "29"
 [dev-dependencies]
 swactor = { path = "../..", features = ["getrandom"] }
 wat = "1"
+proptest = "1"

crates/wasm-actor/src/actor.rs

@@ -41,12 +41,16 @@ impl ActorInterface for WasmActor {
         };
 
         // 2. Write message bytes into guest memory
-        self.memory.data_mut(&mut self.store)
+        let mem = self.memory.data_mut(&mut self.store);
-            [ptr as usize..(ptr as usize + bytes.len())]
+        let end = (ptr as usize).saturating_add(bytes.len());
-            .copy_from_slice(bytes);
+        if end > mem.len() {
+            return; // alloc returned OOB pointer — drop message
+        }
+        mem[ptr as usize..end].copy_from_slice(bytes);
 
         // 3. Call guest handle
         if self.handle.call(&mut self.store, (ptr, len)).is_err() {
+            self.store.data_mut().outbox.clear(); // discard sends from incomplete operation
             return; // handle trapped — drop message, keep actor alive
         }