refactor: optimize gas usage across contracts

[ScriptedBro]

Closes #12
Issue: #12

Summary

This PR implements a gas/cost-efficiency pass across the RemitWise Soroban contracts, focusing on benchmarked hot paths and storage-heavy read operations.

Key improvements

• remittance_split

  • Removed duplicate split arithmetic
  • Added internal split calculation path for distribute_usdc to avoid unnecessary event/allocation overhead
  • Reduced redundant nonce helper cloning

• savings_goals

  • Added owner->goal index cache (OWN_GOAL)
  • Optimized get_all_goals(owner) with a hybrid strategy (index path for subsets, full-scan fast path when owner owns all goals)

• bill_payments

  • Added cached per-owner unpaid totals (UNPD_TOT)
  • Optimized get_total_unpaid(owner) to use cached totals (fallback scan preserved)

• insurance

  • Added cached per-owner active premium totals (PRM_TOT)
  • Optimized get_total_monthly_premium(owner) to use cached totals (fallback scan preserved)

• family_wallet

  • Removed redundant signers.clone() in configure_multisig

• Code cleanup / size-related

  • Removed unused dead file bill_payments/src/schedule.rs
  • Removed stale commented-out schedule module scaffolding

Acceptance Criteria

• Gas usage analyzed
• Optimizations implemented
• Gas benchmarks
• Documentation updated

Gas Benchmark Results (Before vs After)

Aggregate CPU improvement across benchmarked methods: -3.87%

Validation Performed

cargo check -p remittance_split -p savings_goals -p bill_payments -p insurance -p family_wallet

cargo test -p remittance_split -p savings_goals -p bill_payments -p insurance -p family_wallet

cargo test -p remittance_split --test gas_bench -- --nocapture
cargo test -p savings_goals --test gas_bench -- --nocapture
cargo test -p bill_payments --test gas_bench -- --nocapture
cargo test -p insurance --test gas_bench -- --nocapture
cargo test -p family_wallet --test gas_bench -- --nocapture

Documentation

• Added docs/gas-optimization.md with:
  • benchmark methodology/results
  • compiler optimization settings verification
  • current WASM artifact sizes
  • validation commands and tradeoffs

[Baskarayelu]

@ScriptedBro resolve the conflicts & fix the pipeline