Module concurrent

Concurrent effect — spawn, await, cancel, timeout.

Sub-effect of IO. The broader concurrency model (Mutex / RwLock / Atomic / channels) lives in silo:std.sync.

The current shape is a flat +Concurrent with AnyUInt task handles and AnyInt durations. The typed split into +Executor / +Thread / +Concurrent with a typed (Handle val) return arrives once associated effect types and region-scoped local records are wired.