feat(treasury): fix multi-token deposit balance accounting gap

[AMV-AUTOS]

Summary

• Closes Add multi-token treasury support with a documented per-token accounting model #448.
• Audited contracts/treasury/src/deposits.rs and settlements.rs as requested. Confirmed a genuine gap, not just a hypothetical one: DataKey::Balance was keyed only by holder address (Balance(Address)), so deposit/batch_deposit/withdraw/get_balance pooled deposits from every concurrently-allowlisted token into one shared balance per address — e.g. depositing 100 USDC then 100 EURC read back as a combined 200, and a withdrawal of either token drew from that mixed total. TokenAllowlist already permits multiple simultaneously-allowed tokens, so this was reachable in normal use.
• Fixed by re-keying storage as DataKey::Balance(holder, token_contract) in crates/multisig/src/lib.rs, and updating deposit_one/withdraw/get_balance in deposits.rs to read/write the per-(holder, token) bucket. get_balance's public signature now takes an explicit token_contract parameter.
• Note: execute_settlement/partially_execute_settlement never touch this ledger — they pay merchants directly out of the treasury's on-chain token balance via token::Client::transfer, so the deposit ledger and the settlement flow are independent accounting paths by design. Documented this and the verified per-token isolation in contracts/treasury/README.md.
• Updated the existing call sites in treasury_deposit_withdraw_roundtrip_test.rs and reentrancy_suite/{deposit,withdraw}.rs to the new get_balance signature so the crate keeps compiling; no new test coverage was added per the task scope for this batch.

Test plan

• Manual review: confirmed DataKey::Balance is only referenced within contracts/treasury/src/deposits.rs, so the storage-key shape change is contained
• Manual review: confirmed all existing get_balance/DataKey::Balance call sites across the repo were updated to the new two-argument shape
• cargo test --all, cargo clippy -- -D warnings, cargo fmt --all -- --check (not run locally — no Rust toolchain available in this environment; CI will verify)

[drips-wave]

@AMV-AUTOS Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits