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

examples/backend-api/main.nx

@@ -1,30 +1,18 @@
-use core.env
-use db.postgres.Pool
-use web.http.{App, Response}
+struct AppConfig:
+    port: Int
+    service_name: String
 
-struct AppState:
-    pool: Pool
+fn build_message(name: String, port: Int) -> String:
+    if port > 0 && port < 65536:
+        return name
+    else:
+        return "invalid"
 
-async fn health(state: AppState) -> Response:
-    let version = env.get("APP_VERSION").or("dev")
-    let row = await state.pool.query_one<Map>(
-        "select now() as now"
-    )?
-
-    Response.json({
-        "status": "ok",
-        "version": version,
-        "database_time": row["now"]
-    })
-
-async fn main() -> Result<Void, AppError>:
-    let database_url = env.require("DATABASE_URL")?
-    let port = env.get("PORT").or("8080").to_int()?
-    let pool = Pool.connect(database_url, max: 16)?
-
-    let app = App.new()
-        .state(AppState { pool: pool })
-        .get("/health", health)
-
-    await app.listen("0.0.0.0", port)?
+fn main() -> Int:
+    let config = build_message("backend-api", 8080)
+    let enabled = true || false
+    if enabled:
+        return 0
+    else:
+        return 1