Harden payment and refund idempotency

[mdabubakkarhq]

Summary

• Fail closed when an atomic payment-settlement claim is unavailable, and repeat duplicate/reuse checks after acquiring the claim.
• Preserve authoritative foreign-currency capture data when a remote transaction refresh returns only sparse fields.
• Distinguish intentional equal-value partial refunds from incomplete retries, and recover an already-completed full refund before checking the remaining balance.
• Inspect the existing ledger schema before widening columns so normal requests do not repeat DDL.

Validation

• Private regression suite: 101/101 passed (the temporary test harness is intentionally excluded from the public release).
• PHP syntax: 19/19 files passed on PHP 8.3.
• GitHub Actions workflow: actionlint passed.
• Scoped code review: all five release-review findings addressed; no new Critical, Important, or Minor findings.
• Release archive: 26 entries / 25 files, 44,362 bytes.
• SHA-256: 9DF622EB9F5C46472FB52A7D61D00E890D9987EEA52F8417CD3F3451C8ECBF7D.

Release scope

• Version remains 1.0.0 under the MIT License.
• The public repository remains a focused runtime distribution; tests/, scripts/, internal review files, and generated archives are not included.