Add robust property-based tests for the assertNoTimeWarp function using
the rapid testing library. The tests verify the following scenarios:
- Basic property tests:
- Only retarget blocks (block height divisible by blocksPerRetarget) are checked
- Valid timestamps (within maxTimeWarp of previous block) pass validation
- Invalid timestamps (too early) fail with appropriate ErrTimewarpAttack
- Correct boundary behavior (exactly at maxTimeWarp limit)
- Invariant tests:
- Function never panics with valid inputs
- Non-retarget blocks always return nil regardless of timestamps
- Security tests:
- All retarget blocks are protected from timewarp attacks
- Non-retarget blocks are not affected by the timewarp check
