Native watcher watching a WASM actor does not receive ActorExited when
the target is stopped via rt.stop_actor(). Root cause: StopSignal
interception in tick_all (worker.rs:734) sets stopping=true but does
not push to the deaths vector, so phase 5b watch notifications never
fire for externally-stopped actors.
Also adds P0-2 through P1-7 test scenarios (all passing):
- empty message, oversized message, allocator exhaustion
- nonexistent address send, wrong export signature/name
- graceful stop, negative payload_len, independent stores
- WASM-to-WASM relay
Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski
27f417754b