fix(checkout): make split checks atomic and allocation-safe

[khaira777]

Summary

Split-check creation validates source state outside the transaction and uses proportional rounding that can produce an invalid negative remainder for small totals.

Current behavior

• main/routes/bills.ts:263-339 reads and validates the source bill before withTxn.
• Concurrent split requests can both pass validation and create multiple split groups.
• The final check receives totalMinor - used; rounded allocations can make this negative.
• void_adjustment rows are not excluded from item allocation.

Expected behavior

A split operation should be atomic, repeat-safe, and allocate non-negative integer minor-unit amounts whose sums reconcile exactly.

Acceptance criteria

• Move source-state validation and allocation reads into the transaction.
• Allow only one split group per source order/bill.
• Use a largest-remainder or equivalent integer-minor-unit allocation algorithm.
• Reject or exclude invalid/void-adjustment rows.
• Add concurrent split, one-cent total, and voided-item regression tests.

Relevant files

main/routes/bills.ts, split-check integration tests