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

crates/nxc-frontend/tests/lexer_tests.rs

@@ -0,0 +1,48 @@
+use nxc_frontend::{Keyword, Lexer, TokenKind};
+
+#[test]
+fn lexes_literals_and_operators() {
+    let result = Lexer::new("let value = 12.5 + 3 * -2 && true\n").lex();
+    assert!(result.diagnostics.is_empty());
+
+    let kinds: Vec<TokenKind> = result.tokens.into_iter().map(|token| token.kind).collect();
+    assert_eq!(
+        kinds,
+        vec![
+            TokenKind::Keyword(Keyword::Let),
+            TokenKind::Identifier("value".into()),
+            TokenKind::Equal,
+            TokenKind::Float("12.5".into()),
+            TokenKind::Plus,
+            TokenKind::Integer("3".into()),
+            TokenKind::Star,
+            TokenKind::Minus,
+            TokenKind::Integer("2".into()),
+            TokenKind::AndAnd,
+            TokenKind::Bool(true),
+            TokenKind::Newline,
+            TokenKind::Eof,
+        ]
+    );
+}
+
+#[test]
+fn lexes_indent_and_dedent_tokens() {
+    let source = "fn main() -> Int:\n    let value = 1\n    return value\n";
+    let result = Lexer::new(source).lex();
+    assert!(result.diagnostics.is_empty());
+
+    let kinds: Vec<TokenKind> = result.tokens.into_iter().map(|token| token.kind).collect();
+    assert!(kinds.iter().any(|kind| matches!(kind, TokenKind::Indent)));
+    assert!(kinds.iter().any(|kind| matches!(kind, TokenKind::Dedent)));
+}
+
+#[test]
+fn reports_unterminated_string() {
+    let result = Lexer::new("let value = \"oops\n").lex();
+    assert_eq!(result.diagnostics.len(), 1);
+    assert!(result.diagnostics[0]
+        .message
+        .contains("unterminated string literal"));
+}
+