Round up batched withdrawal miner fee splits

[QiLOL]

Summary

Fixes rounding up the per-request miner fee deduction for batched withdrawals.

Previously, new_withdrawal_txn used floor division when validating each withdrawal output:

miner_fee / request_count

For a 1001 sat miner fee split across 3 requests, this charged each user 333 sats, so users only covered 999 sats total. The remaining 2 sats were implicitly absorbed by the bridge-controlled value.

This PR changes the Move-side validation to round up the per-request fee whenever the miner fee is not evenly divisible by the request count. This matches the conservative off-chain construction behavior and ensures users cover the fee remainder instead of the bridge subsidizing it.

Changes

• Round up per_user_miner_fee in withdrawal_queue::new_withdrawal_txn.
• Update the batched-with-remainder withdrawal queue test to use the ceil fee split.

Note: This is a conservative fix: it prevents protocol value from covering fee remainders, but it can overcharge users by up to request_count - 1 sats per batch. An alternative is to keep the base floor split and assign the remainder deterministically to one request, for example the last request pays floor(miner_fee / request_count) + remainder. That would make the total user-paid fee exactly match the miner fee. Feedback welcome on which policy is preferred.