`execute` readiness check compares against a stored `execute_after` but uses deliverable arithmetic only

[therealjhay]

ProblemSettlement execute uses execute_after but the timing source (ledger time vs wall clock) is not uniformly defined between settlement and common scheduling.

1. Timing source ambiguity.
2. Off-by-a-ledger drift.
3. Cross-function inconsistency.

Solution

• Standardize the timing source.
• Test boundaries.

Acceptance Criteria

• Single timing source documented/tested.
• cargo test --workspace passes.

Note for Contributors: Write a clear PR description. Show the timing-boundary test.

[Chigybillionz]

Dear Maintainer,

If you give me the opportunity to work on this task, I'll make sure to resolve it according to the requirements and submit a quality PR before the end of the wave. I won't let you down.

Thank you.

[grantfox-oss]

🦊 GrantFox — @Chigybillionz has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #576)
2. Your PR will be reviewed by the Betta-Pay maintainers

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