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).

examples/1018-errors-multi-value-failable.sx

@@ -26,13 +26,13 @@ inc :: (n: s32) -> (s32, s32, !E) {
 
 // Multi-value `catch`, bare-expression tuple fallback (absorbs the failure).
 safe :: (n: s32) -> s32 {
-    v, b := parse(n) catch e (40, 50);
+    v, b := parse(n) catch (e) (40, 50);
     return v + b;
 }
 
 // Multi-value `catch` match-body — per-tag dispatch, each arm a value-tuple.
 classify :: (n: s32) -> s32 {
-    v, b := parse(n) catch e == {
+    v, b := parse(n) catch (e) == {
         case .Bad:   (1, 1);
         case .Empty: (2, 2);
         else:        (9, 9);