Z3 expression translator fixes:
- implies(x, y) now preserves both arguments (was dropping both → (=> true true))
- Multi-level field access m.price.amount → m_price_amount (was m_price.(>= amount 0))
- len() now handled same as length() (was left as bare len())
- nil/null translated to speckl_nil sentinel constant
- != now properly converts to (distinct X Y) via convertToPrefix (was bare 'distinct')
- Added 'distinct' as binary operator in convertToPrefix
- speckl_nil declared as Int constant in output
All 8 product spec constraints now produce valid SMT-LIB2.
57/57 tests pass.
