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/1054-errors-backtick-reserved-binding.sx

@@ -16,7 +16,7 @@ parse :: (n: s32) -> (s32, !E) {
 
 // `catch` tag binding spelled `s2`, referenced in the match body.
 classify :: (n: s32) -> s32 {
-    return parse(n) catch `s2 == {
+    return parse(n) catch (`s2) == {
         case .Bad:   1;
         case .Empty: 2;
         else:        3
@@ -25,7 +25,7 @@ classify :: (n: s32) -> s32 {
 
 // `onfail` tag binding spelled `u8`, referenced in the cleanup body.
 cleanup :: (n: s32) -> !E {
-    onfail `u8 { if `u8 == error.Bad { print("cleanup: bad\n"); } }
+    onfail (`u8) { if `u8 == error.Bad { print("cleanup: bad\n"); } }
     if n < 0 { raise error.Bad; }
     return;
 }