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-cli/Cargo.toml

@@ -5,6 +5,10 @@ edition.workspace = true
 license.workspace = true
 authors.workspace = true
 
+[[bin]]
+name = "nxc"
+path = "src/main.rs"
+
 [[bin]]
 name = "nexacore"
 path = "src/main.rs"

crates/nxc-cli/src/main.rs

@@ -20,15 +20,28 @@ fn run() -> Result<(), String> {
     };
 
     match command.as_str() {
-        "build" => {
+        "check" | "build" => {
             let Some(path) = args.next() else {
-                return Err("usage: nexacore build <file.nx>".to_string());
+                return Err(format!("usage: {} {command} <file.nx>", executable_name()));
             };
-            let result = nxc_driver::compile_file(Path::new(&path))
-                .map_err(format_compile_error)?;
-            println!("compiled {path}");
-            println!("tokens: {}", result.tokens.len());
-            println!("items: {}", result.module.items.len());
+
+            let output =
+                nxc_driver::check_file(Path::new(&path)).map_err(format_driver_error)?;
+
+            if output.has_errors() {
+                eprintln!("{}", output.render_diagnostics());
+                return Err(format!(
+                    "check failed with {} diagnostic(s)",
+                    output.diagnostics.len()
+                ));
+            }
+
+            let summary = output.summary();
+            println!("checked {}", output.path.display());
+            println!("tokens: {}", output.tokens.len());
+            println!("items: {}", summary.items);
+            println!("functions: {}", summary.functions);
+            println!("structs: {}", summary.structs);
             Ok(())
         }
         "run" => Err("runtime execution is not implemented yet".to_string()),
@@ -41,25 +54,34 @@ fn run() -> Result<(), String> {
     }
 }
 
-fn format_compile_error(error: nxc_driver::CompileError) -> String {
+fn executable_name() -> String {
+    env::args()
+        .next()
+        .and_then(|path| {
+            Path::new(&path)
+                .file_name()
+                .map(|name| name.to_string_lossy().to_string())
+        })
+        .unwrap_or_else(|| "nxc".to_string())
+}
+
+fn format_driver_error(error: nxc_driver::DriverError) -> String {
     match error {
-        nxc_driver::CompileError::Io(io) => format!("io error: {io}"),
-        nxc_driver::CompileError::Parse(parse) => format!(
-            "parse error at line {}, column {}: {}",
-            parse.span.line, parse.span.column, parse.message
-        ),
+        nxc_driver::DriverError::Io(io) => format!("io error: {io}"),
     }
 }
 
 fn print_help() {
+    let name = executable_name();
     println!("NexaCore CLI");
     println!("usage:");
-    println!("  nexacore build <file.nx>");
-    println!("  nexacore run <file.nx>");
-    println!("  nexacore new <name>");
-    println!("  nexacore test");
-    println!("  nexacore fmt");
-    println!("  nexacore add <package>");
-    println!("  nexacore doc");
+    println!("  {name} check <file.nx>");
+    println!("  {name} build <file.nx>");
+    println!("  {name} run <file.nx>");
+    println!("  {name} new <name>");
+    println!("  {name} test");
+    println!("  {name} fmt");
+    println!("  {name} add <package>");
+    println!("  {name} doc");
 }