Define GC pause and progress contract

[andrewtdiz]

Summary

• Define Perry's low-pause GC progress contract and bounded budget model.
• Add telemetry shape for progress kind, budget unit, configured work budget, and soft pause target.
• Add focused runtime tests that lock the contract before the larger resumable-cycle PRs land.

Verification

running 4 tests
....
test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 746 filtered out; finished in 0.00s

This is the first small PR in the GC runtime improvement train; it is intentionally contract/scaffolding only so the later resumable-cycle and barrier/root PRs have a stable review target.