refactor: centralize runtime logic (v0.4)

Extract scheduler responsibilities into a dedicated Runtime component:
- src/runtime.rs: New centralized control flow (669 lines)
- src/scheduler.rs: Simplified to task queue & preemption management
- tests/runtime.rs: Comprehensive runtime test suite
- benches/multi_scheduler.rs: Multi-runtime scheduling benchmarks
- Improves modularity and enables per-runtime configuration

Cargo.toml

@@ -9,7 +9,7 @@ libc = "0.2"
 
 [dev-dependencies]
 libc = "0.2"
-tokio = { version = "1", features = ["rt", "macros", "sync"] }
+tokio = { version = "1", features = ["rt", "rt-multi-thread", "macros", "sync"] }
 
 [profile.dev]
 panic = "unwind"
@@ -22,3 +22,7 @@ codegen-units = 1
 [[bench]]
 name = "primes"
 harness = false
+
+[[bench]]
+name = "multi_scheduler"
+harness = false