[Enhancement] Add env.advance_sequence_and_time() combined helper for ledger epoch simulation

[ekelemepraise-code]

Summary

Advancing ledger time and sequence number separately requires two calls:

env.advance_time(Duration::days(7));
env.advance_sequence(1000);

In practice, Stellar ledger sequences and timestamps are coupled. A combined helper would make epoch simulation more realistic and concise.

Proposed API

// Advance both time and sequence atomically
env.advance_epoch(Duration::days(7), 1000);

// Or with a named struct for clarity
env.advance_epoch(LedgerEpoch {
    duration: Duration::days(7),
    sequences: 1000,
});

Rationale

Each Stellar ledger closes roughly every 5 seconds, so 7 days ≈ ~120,960 ledgers. The helper could optionally auto-calculate sequence count from duration based on a configurable close time.

Acceptance Criteria

• advance_epoch(duration, sequences) implemented in contracts/crucible/src/time.rs
• Optional with_ledger_close_time(seconds) builder method for auto-calculation
• Exported via prelude
• Tests added for edge cases (zero duration, zero sequences)