Add fuzz/property testing for lexer and parser

### Title
Introduce fuzz testing for lexer and parser
### Description
Compiler frontends benefit heavily from fuzzing.

- We should:
- Feed random inputs
- Ensure no crashes
- Validate error stability

### Acceptance Criteria

- [ ]  Fuzzing setup (libFuzzer or similar)
- [ ]  Corpus of valid/invalid inputs
- [ ]  CI integration (optional)