Sixteen small, isolated contracts.

Unique case catalog

1. U001 constructor exposes the documented empty state
2. U002 empty add returns the receiver without scheduling
3. U003 empty next is an idle no-op
4. U004 a selected task receives its Stack as this
5. U005 task return values do not perform a hand-off
6. U006 instances keep separate live storage
7. U007 contents preserves a supplied replacement array
8. U008 stack alias preserves a supplied replacement array
9. U009 invalid add batches are rejected atomically
10. U010 contents rejects non-array replacement values
11. U011 stack setter rejects a nonfunction member
12. U012 source preparation requires the canonical package import
13. U013 source preparation maps the package import to checked-in source
14. U014 nonempty add returns the receiver
15. U015 contents replacement participates in the next hand-off
16. U016 stack alias replacement participates in the next hand-off

Run this suite

npm run test:unit

The same host-neutral definitions are reused by the Node 22.13/24 matrix and native coverage entry; those replays do not create new IDs.