Fix get freshness lag calculation

[samlogy1]

closes #1541

📝 Description

Adds a test suite for get_freshness lag calculations (Issue #1541). Covers lag at zero, lag at positive, and lag during pause. All tests use plain #[cfg(test)] — no feature gate — so they run on every CI matrix entry.

🎯 Type of Change

• Other: Test coverage — get_freshness lag calculation lock-in

🔧 Changes Made

Files Modified

• quicklendx-contracts/src/lib.rs — registered new test module

New Files Added

• quicklendx-contracts/src/test_freshness_lag.rs — 13 tests for lag at zero, positive, and during pause

Key Changes

• Zero lag when indexed timestamp matches current ledger time
• Positive lag equals exact elapsed seconds (current − indexed)
• get_freshness remains callable while paused and returns correct lag
• Lag accumulates correctly during pause and is not reset by unpause
• Drift boundary: 120 s = fresh, 121 s = stale
• Sad path: future indexed timestamp returns negative lag (clock skew), not stale

🧪 Testing

• Unit tests pass
• No breaking changes introduced
• Edge cases tested

Test Coverage

13 deterministic tests — no Date.now() or random values. Ledger timestamp is fixed in every test via env.ledger().set_timestamp().

📋 Contract-Specific Checks

• Soroban contract builds successfully
• WASM compilation works
• Contract functions tested
• Access control verified

Contract Testing Details

Pause tests call initialize_admin then client.pause(&admin) to exercise the real pause path. No std:: calls introduced. #![no_std] discipline preserved.

🔍 Code Quality

• Clippy warnings addressed
• No unused imports or variables
• Complex logic is commented

🔗 Related Issues

Closes #1541

🧪 How to Test

1. Run the new tests:
2. Run Clippy:
3. Verify WASM build:

⚠️ Breaking Changes

None. Only new test code added — no production logic or APIs modified.

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[Baskarayelu]

Fix get freshness lag calculation — great addition. merging 👍