[documentation] v2 design: streaming bounty payouts (PRD NG3, deferred)

[yinkscss]

Context

PRD non-goal NG3 excludes streaming payments (continuous DCA). v1 pays discrete amounts per merged PR.

Problem

Feature requests for vesting or streaming rewards need a deferred design reference.

Proposed scope

• Add docs/future/streaming-payouts.md with use cases and Soroban constraints
• Document why v1 uses immediate transfer in record_merge
• Note relationship to Drips Wave discrete issue payouts

Acceptance criteria

• Document marked v2 / out of scope for v1
• Explains fail-closed full payout model (PRD OQ7)
• No implementation work in v1 crates

References

• docs/PRD.md NG3, OQ7
• contracts/waveflow-escrow/src/contract.rs (record_merge transfer)