feat: full runtime redesign (v0.6)

Complete rewrite with improved architecture & correctness:
- src/runtime.rs: Simplified task scheduling with proper state transitions
- src/scheduler.rs: Decoupled from runtime, pure task queue logic
- src/io.rs, src/mutex.rs: Refactored for clarity & performance
- New actor model framework (src/actor.rs, src/context.rs)
- Channel primitives (src/channel.rs) & process IDs (src/pid.rs)
- Preemption framework (src/preempt.rs) for fair timeslicing
- Expanded benchmarks & tests (multi_scheduler, primes, runtime)

.gitignore

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+/target
+Cargo.lock