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/1038-errors-comptime-run-handled.sx

@@ -20,10 +20,10 @@ guard :: (ok: bool) -> !E {
 }
 
 ok_v   :: #run parse(5);            // success → 10 (value, error stripped)
-caught :: #run parse(-1) catch e 99; // Bad   → 99
+caught :: #run parse(-1) catch (e) 99; // Bad   → 99
 ored   :: #run parse(0) or 55;       // Empty → 55
 
-#run guard(false) catch e { };       // onfail fires during the comptime unwind
+#run guard(false) catch (e) { };       // onfail fires during the comptime unwind
 
 main :: () -> s32 {
     print("ok={} caught={} ored={}\n", ok_v, caught, ored);