Add a cargo-fuzz harness exercising the runtime under arbitrary actor spawn/send/inbox sequences in both single- and multi-threaded modes.
- fuzz/Cargo.toml: add an isolated `swactor-fuzz` package (own standalone workspace) wiring libfuzzer-sys + arbitrary against the `swactor` crate, with `fuzz_runtime` and `fuzz_mt` bin targets
- fuzz/fuzz_targets/fuzz_runtime.rs: single-threaded model target running randomized scenarios over Echo/Counter/Forwarder/Spawner/Bomber/Noop/WrongType actors with shared-inbox draining and invariant checks
- fuzz/fuzz_targets/fuzz_mt.rs: multi-threaded target driving a live `Runtime`/`RuntimeHandle` across 2-4 threads with spawn/send/burst/inbox actions and `check_invariants` (worker/actor counts) after each run
- fuzz/fuzz_targets: both targets derive a `FuzzInput`/`Action` model from `arbitrary`, cap the action count, and support opt-in run tracing via the `FUZZ_LOG` env var
- .gitignore: ignore fuzz corpus and artifact output
Signed-off-by: Zachery Aaron Shores-Chmielewski <zacheryasc@gmail.com>
