Refactor: deduplicate is_settleable into single implementation

[aizyohiy-hash]

Closes #458

Summary

This PR refactors the escrow contract to ensure that the settlement predicate logic is consolidated into a single, authoritative implementation backed by a private helper, keeping the codebase MVP-ready, easy to review, and safe for audit.

Changes Made

• Canonical Helper: Extracted the settlement predicate logic into a single private helper: fn settleable_now(env: &Env) -> bool.
• Single Public View: The public pub fn is_settleable(env: Env) -> bool now solely delegates to the private helper.
• Removed Duplicates: Verified that no duplicate implementations or logic remain in the codebase.
• Test Coverage: Added focused, state-matrix tests to contracts/escrow/src/tests/settlement.rs to verify that is_settleable accurately reflects funded state, maturity, and legal holds.
• Documentation: Added concise Rust doc comments explaining the settlement requirements.

Security & Behavior Invariants

• No ABI/Export Changes: The observable behavior, function signature, visibility, and exported WASM symbol remain exactly the same.
• Gates Unchanged: The maturity, funded, and legal-hold gates are strictly enforced without any relaxation, reordering, or caching.
• Storage Layout: No changes were made to the storage layout.

Verification

• Code strictly follows the formatting guidelines.
• cargo build completes successfully.
• cargo test passes all tests, including the new settlement coverage.

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[mikewheeleer]

nice work — merging this 🙌