lexer + parser + codegen: try/catch statement

try { ... } catch (name: Type) { ... } is now a first-class El statement.

Lexer: `try` and `catch` are now keywords (Try, Catch token kinds).
Parser: TryCatch AST node with try_body, catch_name, catch_body.
codegen-js: emits try { ... } catch (name) { ... } directly -- correct
  for all browser error handling patterns.
codegen.el (C backend): emits the try body with a comment; exception
  handling is a no-op since C has no analogous mechanism. Programs using
  try/catch should compile with --target=js.

The catch variable type annotation is parsed and skipped (same treatment
as all other type annotations in El).

el-compiler/src/codegen.el

@@ -1143,6 +1143,15 @@ fn cg_stmt(stmt: Map<String, Any>, indent: String, declared: [String]) -> [Strin
     if kind == "Import"  { return declared }
     if kind == "ExternFn" { return declared }
     if kind == "CgiBlock" { return declared }
+    // TryCatch: browser-only control flow. In the C target, emit a comment
+    // noting that the try body runs unconditionally; error handling is a no-op.
+    // Programs that rely on catching JS exceptions should compile with --target=js.
+    if kind == "TryCatch" {
+        let try_body = stmt["try_body"]
+        emit_line(indent + "/* try (C target: exception handling not supported) */")
+        cg_stmts(try_body, indent, native_list_clone(declared))
+        return declared
+    }
     declared
 }