feat(parser): optimize string concatenation assignment operations
- Preserve PHP's concat-assignment operation for statically typed strings using in-place append
- Add parseInPlaceStringConcatAssign method to handle string concatenation assignments efficiently
- Prevent O(n^2) work when rebuilding target = concat(target, rhs) operations
- Keep compound RHS evaluated completely before target changes in concat assignments
- Optimize native scalar binary operand handling to avoid unnecessary Variant boxing
- Add native scalar call results remain unboxed when ordered in native-types mode
- Create string concatenation assignment tests with proper PHP value and COW semantics
- Update version number from 1109 to 1110
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