Implements early exit optimization for call sequence shrinking to avoid
unnecessary shrinking attempts when sequences are already minimal.

Changes:
- Add canShrinkTransaction() to check if a single transaction can be shrunk
- Add canShrinkFurther() to check if a sequence has shrinking potential
- Integrate early exit check in shrinkCallSequence() main loop
- Refactor shrinking helpers into fuzzer_worker_shrinking.go
- Simplify removeReverts() to avoid delay accumulation bugs

The shrinking loop now exits early when:
- Sequence has only 1 transaction AND
- That transaction has zero value, gas price, and delays

Toward #752

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>