test: add property-based testing and fuzzing for Actor

Add proptest dependency for property-based tests.
Add cargo-fuzz target for Actor operations:
- Test random sequences of request/message operations
- Verify state consistency across all backends
- Run with: cd concurrency/fuzz && cargo fuzz run fuzz_actor_operations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: unbalancedparentheses

concurrency/Cargo.toml

@@ -16,4 +16,4 @@ pin-project-lite = "0.2"
 # This tokio imports are only used in tests, we should not use them in the library code.
 tokio-stream = { version = "0.1.17" }
 tokio = { version = "1", features = ["full"] }
-proptest = "1.5"
+proptest = "1.9.0"

concurrency/fuzz/.gitignore

@@ -0,0 +1,4 @@
+target
+corpus
+artifacts
+coverage