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/parser.el

@@ -912,6 +912,40 @@ fn parse_stmt(tokens: [Map<String, Any>], pos: Int) -> Map<String, Any> {
         return make_result({ "stmt": "For", "item": item_name, "list": list_expr, "body": body }, p)
     }
 
+    // try/catch statement
+    // try { body } catch (name: Type) { handler }
+    // The catch variable name and type are both captured; type is skipped.
+    if k == "Try" {
+        let p = pos + 1
+        let r_try = parse_block(tokens, p)
+        let try_body = r_try["stmts"]
+        let p = r_try["pos"]
+        let catch_name = "err"
+        let k2 = tok_kind(tokens, p)
+        if str_eq(k2, "Catch") {
+            let p = p + 1
+            let p = expect(tokens, p, "LParen")
+            // catch variable name
+            let kn = tok_kind(tokens, p)
+            if str_eq(kn, "Ident") {
+                let catch_name = tok_value(tokens, p)
+                let p = p + 1
+            }
+            // optional type annotation: : Type
+            let k3 = tok_kind(tokens, p)
+            if str_eq(k3, "Colon") {
+                let p = p + 1
+                let p = skip_type(tokens, p)
+            }
+            let p = expect(tokens, p, "RParen")
+            let r_catch = parse_block(tokens, p)
+            let catch_body = r_catch["stmts"]
+            let p = r_catch["pos"]
+            return make_result({ "stmt": "TryCatch", "try_body": try_body, "catch_name": catch_name, "catch_body": catch_body }, p)
+        }
+        return make_result({ "stmt": "TryCatch", "try_body": try_body, "catch_name": catch_name, "catch_body": native_list_empty() }, p)
+    }
+
     // @decorator — capture decorator name and attach to following stmt
     if k == "At" {
         let p = pos + 1