Add explicit async composition primitives

Add explicit async composition primitives复制锚点链接

Add independent limit, retry, singleflight, and wait subpaths without runtime dependencies. Limit shared or per-key concurrency, explicitly authorize bounded retries, share only in-flight work, and cancel waits using native AbortSignal. Stateful handles close admission and drain accepted work; cancelled waiters do not acquire ownership of shared operations.

Extend iter with sequential forEach, find, some, every, and reduce consumers that preserve upstream cleanup and drain semantics. Preserve the existing grfn and iter contracts. Generate all source and published exports with tsdown and validate behavior, public types, and source-free consumption.

Singleflight accepts the operation on run(key, task), sharing the first pending task for each key. Add read-only get / size and snapshot drain so domain coordinators can enforce capacity and mutation ordering without maintaining another task registry. Migrate the workspace Cache coordinator and all async examples to this contract.