feat(remittance): add signed split allocation quotes

[jamilahmadzai]

Summary

Closes #1011.

Adds an authenticated allocation-quote path for remittance_split:

• Adds get_split_allocations_hash(caller, total_amount, deadline) so callers can compute the exact quote hash off-chain.
• Adds get_split_allocations_signed(caller, total_amount, deadline, request_hash) to validate amount, deadline, hash binding, and caller authorization before returning allocations.
• Binds the quote hash to the shared signing domain, alloc functional tag, network id, current contract address, caller address, total amount, and deadline.
• Delegates the final allocation calculation to the existing get_split_allocations path so signed and unsigned quote semantics stay in lockstep.

Security notes

• The signed quote path performs no token transfers and writes no contract state.
• The auth payload includes the caller, network, contract instance, amount, and deadline.
• The request hash uses stable XDR address bytes for the caller and contract address to avoid object-id instability across repeated computations.
• Deadline handling mirrors the public hashed distribution semantics: deadline <= now expires, deadline > now + MAX_DEADLINE_WINDOW_SECS is invalid, and the exact max-window boundary is accepted.

Tests

• cargo test -p remittance_split test_get_split_allocations_signed -- --nocapture
• cargo clippy -p remittance_split --all-targets completed with existing warnings.
• git diff --check

Known baseline: the full cargo test -p remittance_split -- --nocapture suite is currently red on existing tests unrelated to this change. The failures include older tests still initializing splits with 50/30/15/5-style values while initialize_split now validates basis points summing to 10_000, plus an existing event-topic assertion. The focused tests added for this issue pass.

Bounty: GrantFox OSS campaign for issue #1011.

[jamilahmadzai]

Checked the failing CI jobs after opening this PR. The failures are outside the touched scope:

• Build and Test stops while compiling reporting/src/lib.rs / reporting/src/utils.rs (ContractError import/derive issues, to_i128_checked trait import, and existing ?/return-type errors).
• Lint (unwrap/expect ban) fails on the same reporting compile errors before reporting any issue in remittance_split.
• Validate and Build fails on the same unrelated reporting crate errors.
• Gas Benchmarks, Security Checks, and Storage Key Naming Convention Validation pass.

This PR only changes remittance_split/src/lib.rs and remittance_split/src/test.rs. The focused validation for the new signed allocation quote path still passes locally:

cargo test -p remittance_split test_get_split_allocations_signed -- --nocapture
git diff --check