feat: deterministic split rounding fairness

[menawar]

Summary

• implement largest-remainder rounding with deterministic tie-break order (spending, savings, bills, insurance) to eliminate drift
• keep split sum exactly equal to total_amount for all inputs
• update tests (unit + gas/bench) and refresh snapshots document the new rounding policy

Testing-

cargo test -p remittance_split

Notes- snapshots updated due to rounding policy change

Closes #246

[Baskarayelu]

@menawar Please resolve the conflicts