Expose package-owned worker jobs

[peter-jerry-ye]

Why

Packages outside async need to offload blocking native or runtime work without duplicating event-loop notification and worker-pool lifecycle. The existing job type is internal, so those packages cannot submit their own opaque jobs through the shared scheduler.

What

• Add a small moonbitlang/async/job package exposing an opaque, target-specific Job and a with_job submission interface.
• Keep native job construction in the binding package and allow Wasm hosts to provide opaque job handles.
• Run completion callbacks on the event-loop thread while the job and its structured result remain valid.

Why this shape

The public facade and event loop share the same Job type through a type-only leaf package. Native binding functions can therefore return the scheduler's exact opaque Job type directly, without exposing the event-loop package or requiring a representation cast. Wasm handle lowering remains private to the event-loop implementation.

The event loop learns only about an opaque job. It does not learn package-specific operations, result types, or concurrency policy. Notification, worker lifecycle, cancellation, and completion remain in their existing owners. Resource-specific coordination belongs to the submitting package.

[coveralls]

Coverage Report for CI Build 1080

Coverage decreased (-0.02%) to 76.954%

Details

• Coverage decreased (-0.02%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (4 of 5 lines covered, 80.0%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
src/job/types/job.wasm.mbt	1	0	0.0%
Total (3 files)	5	4	80.0%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
src/internal/event_loop/process_unix.mbt	1	44.44%

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Coverage Stats

Relevant Lines:	4938
Covered Lines:	3800
Line Coverage:	76.95%
Coverage Strength:	12015.75 hits per line

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💛 - Coveralls