Epic 1: Diagnostics & UX (front-end polish)

## Epic 1: Diagnostics & UX (front-end polish)

### Goal: compiler is pleasant to use, errors are actionable, multiple errors per run.

- [ ] Standardize a diagnostic model: every diagnostic has `(file, line, col, message, severity)`
- [ ] Print source snippet + caret underline (and ideally a range)
- [ ] Ensure *all* errors have column info (not just line)
- [ ] Implement error recovery:
- [ ] Parser sync points (e.g., `;`, `}`, `)` depending on grammar)
- [ ] Continue semantic analysis when safe (collect errors, avoid noisy cascades)
- [ ] Add warnings:
- [ ] unused variables / unused `let` bindings
- [ ] unreachable code after `return`
- [ ] shadowed bindings
- [ ] Add `--warnings-as-errors` and `--no-warn` flags
- [ ] Add `--verbose` flag (show which phase is running, file being processed)