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/1048-errors-cleanup-absorption.sx

@@ -13,7 +13,7 @@ recover :: () -> (s32, !E) { raise error.Bad; }
 
 work :: (n: s32) -> !E {
     defer  print("defer: always\n");                                 // plain cleanup
-    onfail { failing() catch e print("onfail: caught (catch)\n"); }  // catch absorbs
+    onfail { failing() catch (e) print("onfail: caught (catch)\n"); }  // catch absorbs
     onfail { x := recover() or 7; print("onfail: x={} (or)\n", x); } // or-value absorbs
     if n < 0 { raise error.Bad; }
     return;