Add settlement.simulate claim

[gloskull]

Motivation

• Provide a read-only preview of a developer claim so callers can inspect the outcome of withdraw_developer_balance without performing state changes or transfers.

Description

• Add a typed return struct ClaimSimulation containing developer, recipient, token, current and remaining balances, contract liquidity, daily cap and same-day withdrawal state in contracts/settlement/src/lib.rs.
• Implement simulate_claim(env, developer, amount, to) as a read-only view that reuses the same validation logic as withdraw_developer_balance but performs no auth, transfers, storage writes, TTL updates, or events in contracts/settlement/src/lib.rs.
• Add focused unit tests exercising a successful preview, verifying no side effects, and asserting that validation errors (non-positive amount, overdraw, claim-window closed) are returned by the view in contracts/settlement/src/test.rs.
• Document the new visible API in contracts/settlement/API.md describing the semantics and fields returned by simulate_claim.

Testing

• Added unit tests for simulate_claim under contracts/settlement/src/test.rs but running the package test suite with cargo test -p callora-settlement --lib failed to complete because the build fails in an upstream dependency (soroban-env-host) with a ChaCha20Rng / ed25519_dalek::rand_core::CryptoRng trait mismatch error.
• Ran rustfmt on the modified settlement files successfully, while cargo fmt of the whole repo reported pre-existing parse errors in unrelated tests (in contracts/vault/src/test.rs).
• The new tests should pass once the environment/dependency compile issue and unrelated repo parse errors are resolved.

Closes #571

[greatest0fallt1me]

LGTM ✅ merging. Thanks!