fix: lazy quantifier backtracking in regex engine

The backtracking engine's find() method set a `disable_lazy_backtrack` flag that prevented lazy quantifiers (*?, +?, {m,n}?) from trying more than the minimum matches. This caused patterns like `\$t\((.*?)\)` to fail entirely instead of finding minimal matches.

  Fixes:
  - Remove `disable_lazy_backtrack` flag that broke lazy quantifier semantics
  - Enhance `collectAllMatches` to recurse through concat/group/alternation nodes
  - Add `matchNodeConstrained` to correctly re-set captures when trying
    different quantifier positions
  - Fix capture offset bug in `findAll` for backtracking engine (positions
    were relative to sliced input but used as indices into full input)
  - Re-enable lazy quantifier test suite and fix expectations to match
    standard regex semantics

  Closes #1