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/1012-errors-value-failable-consume.sx

@@ -2,7 +2,7 @@
 // the consumer side of the error-channel tuple ABI). `try f()` on a
 // `-> (T, !E)` callee binds the value slot on success and propagates the error
 // on failure (a pure-failable caller returns the tag; a value-carrying caller
-// returns `{undef, tag}`). `f() catch e BODY` yields the value slot on success
+// returns `{undef, tag}`). `f() catch (e) BODY` yields the value slot on success
 // or the handler body's value on failure, merged through a block parameter.
 // The producer side is `examples/228-value-failable.sx`.
 
@@ -31,12 +31,12 @@ relay :: (n: s32) -> !E {
 
 // value-carrying `catch`, bare-expression fallback.
 safe :: (n: s32) -> s32 {
-    return parse(n) catch e 0;
+    return parse(n) catch (e) 0;
 }
 
 // value-carrying `catch`, match-body value.
 classify :: (n: s32) -> s32 {
-    return parse(n) catch e == {
+    return parse(n) catch (e) == {
         case .Bad:   1;
         case .Empty: 2;
         else:        3