nessi
dfd2f10234
feat: add diagnostic system and improve CLI with check command
Implement comprehensive diagnostic reporting system:
- Add Diagnostic struct with severity levels and span-based error tracking
- Add diagnostic rendering with source context and caret positioning
- Replace ParseError with diagnostic collection in lexer and parser
- Add LexResult and ParseResult types to carry diagnostics
Enhance driver crate with frontend output:
- Replace CompileResult with FrontendOutput containing diagnostics
- Add has_errors() and render_diagnostics() methods
- Add AstSummary for