lang: catch/onfail error bindings take parens

try foo() catch (e) { }   // legal
try foo() catch e { }     // parse error with a migration hint

Same capture style as the for-loop. All four catch shapes keep working
with the parenthesized binding — block, bare-expression body, and the
== match sugar — and the no-binding forms are unchanged. onfail follows
the same rule (onfail (e) { }); its expression-cleanup form is
disambiguated by the paren-group-before-brace lookahead, so
onfail (f()); stays an expression cleanup.

AST unchanged; the printer renders the parens; the #run escape help
text updated. Corpus migrated (57 catch + 3 onfail bindings, in-source
parser test strings, specs incl. grammar rules, readme untouched —
no catch examples there).

Regression: examples/1157-diagnostics-catch-binding-needs-parens.sx;
re-captured stderr for 1010/1013/1037/1123 (migrated source echoed in
carets + help text).

src/ir/emit_llvm.zig

@@ -807,7 +807,7 @@ pub const LLVMEmitter = struct {
                 std.debug.print("  {s} at {s}:{d}:{d}\n", .{ fname, file, line, col });
             }
         }
-        std.debug.print("help: handle it at the `#run` site — `#run <expr> catch e {{ ... }}` or `#run <expr> or <default>`\n", .{});
+        std.debug.print("help: handle it at the `#run` site — `#run <expr> catch (e) {{ ... }}` or `#run <expr> or <default>`\n", .{});
     }
 
     /// Run comptime side-effect functions (e.g., `#run main();` at top level).