feat: implement automated portfolio rebalancing engine

[ChukwuemekaP1]

Summary

This PR introduces a new convenience method, get_score_age(wallet: Address, asset_pair: Symbol) -> u64, to simplify retrieval of the age of a submitted score.

The function calculates the elapsed time since the last score submission using the current ledger timestamp and the stored submission timestamp. If no submission has been recorded, the function returns u64::MAX as a sentinel value to indicate that the score has never been submitted.

Changes Made

• Added get_score_age(wallet: Address, asset_pair: Symbol) -> u64 to lib.rs.
• Implemented sentinel handling by returning u64::MAX when get_last_submit_time() returns 0.
• Used env.ledger().timestamp().saturating_sub(last_submit_time) to safely calculate score age and avoid underflow.
• Preserved compatibility with the existing score submission and staleness-checking logic.

Testing

Added unit tests covering the following scenarios:

• Returns u64::MAX before any score submission exists.
• Returns the correct age after a score is submitted.
• Correctly increases the returned age as ledger time advances.
• Verifies safe behavior when calculating elapsed time using saturating subtraction.

This addition provides a straightforward way for callers to obtain the raw age of a score without performing multiple reads or manually calculating the elapsed time. It complements the existing is_score_stale functionality by exposing the underlying age value directly, improving usability for logging, monitoring, and UI display scenarios.

closes #71