Add hashed split dust conservation tests

[brentthomas248]

Closes #1012

Summary:

• Add a deterministic property-style sweep for distribute_usdc_hashed across generated valid basis-point split configurations.
• Assert the actual recipient token balances and DistributionCompletedEvent payload conserve the full total, including dust-heavy amounts.
• Add invalid percentage rejection coverage for over-100%, under-100%, and over-allocated split configs.

Tests:

• PATH="$HOME/.cargo/bin:$PATH" rustfmt --check remittance_split/src/test.rs
• PATH="$HOME/.cargo/bin:$PATH" cargo test -p remittance_split test_distribute_usdc_hashed -- --nocapture
• git diff --check

Note:

• cargo test -p remittance_split currently has unrelated baseline failures in existing tests that still initialize splits with percentage values like 50/30/15/5 while the contract validates basis points summing to 10000; the new test_distribute_usdc_hashed coverage passes in the full run.