[Onomah1010]

Summary

• Raise the batch payment limit to 100 with a Soroban CPU benchmark and max-size regression test.
• Add validated fee-recipient splits, rounding-safe transfers, and admin-timelocked recipient updates.
• Add a per-contract security audit checklist and external-audit placeholder, and update deployment/migration docs.

Closes #1228
Closes #1229
Closes #1230

Validation

• cargo test -p batch-payments --lib
• cargo test -p fee_distributor --lib
• Batch benchmark: 100 payments used 6,853,384 CPU instructions (68,533 per payment), below the 100,000,000 budget.