fix(tempo): always bind payments to the challenge memo

[raubrey-stripe]

Summary

• PR 27 skipped challenge-bound memo checks whenever a merchant memo was present. TempoMethod.Builder.memo() is method-wide, so distinct challenges A and B advertised the same memo and the first presenter of a matching hash won.
• Always require the MPP attribution memo (realm + challenge id). An explicit memo remains an extra exact-match constraint, not an escape hatch.
• A static Builder.memo() cannot satisfy that binding; omit it unless the advertised memo is unique per challenge. This is the invariant issue 25 needs before replay claims can expire.

Test plan

• ./gradlew test
• Same explicit memo, two challenge IDs: presenting B's hash under A is rejected with memo is not bound to this challenge
• Attribution memo for B still verifies under B and is rejected under A without consuming the replay claim
• Automatic attribution path still accepts matching realm/challenge and rejects wrong realm/challenge, plain Transfer, and replay

Made with Cursor

[raubrey-stripe]

Closing; this is only an issue when a server sets its own explicit memo that replaces a challenge-bound memo that's auto created by the SDK. This issue exists today in mppx, was flagged in wevm/mppx#291 and not fixed.