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/0415-protocols-protocols.sx

@@ -169,7 +169,7 @@ sm_pair :: (a: s32, b: s32) -> (s32, s32, !) {
 
 // `catch` block that diverges (logs the tag, then returns a fallback)
 sm_or_default :: (n: s32) -> s32 {
-    return sm_parse(n) catch e {
+    return sm_parse(n) catch (e) {
         print("  logged {}\n", e);
         return -1;
     };