fix(splitter): add claimable fallback for failed payouts (#260)

[Sam-Rytech]

Description

This PR addresses the issue where payout() pushes to every recipient in a single transaction, causing the entire payout to revert if a single recipient cannot receive the token (e.g. no trustline or unfunded destination).

To resolve this, a claimable fallback has been implemented:

• Uses client.try_transfer() in payout to pre-check transfers for Recipient::Account.
• If the transfer is impossible or fails, it falls back to crediting the token amount to a new AccountBalance in persistent storage rather than reverting.
• Added pub fn claim(...) to the Splitter so the blocked recipient can manually withdraw the funds once their trustline is resolved.
• Follows the conservation of funds (paid + claimable == amount in).

Closes #260