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/1016-errors-onfail.sx

@@ -2,7 +2,7 @@
 // (ERR step E1.7). Unlike `defer` (which runs on every exit), `onfail` fires
 // on an error exit — a `raise` or a propagating `try` — and is skipped on
 // success. On an error exit `defer` and `onfail` run interleaved in reverse
-// declaration order. `onfail e { … }` binds the in-flight error tag.
+// declaration order. `onfail (e) { … }` binds the in-flight error tag.
 // (Per-attempt-`try` gating and `or`-chain absorption refine this in E2.4b.)
 
 #import "modules/std.sx";
@@ -25,7 +25,7 @@ run :: (n: s32) -> !E {
 
 // `onfail e` binds the tag.
 classify :: (n: s32) -> !E {
-    onfail e { if e == error.Bad { print("cleanup: bad\n"); } }
+    onfail (e) { if e == error.Bad { print("cleanup: bad\n"); } }
     if n < 0 { raise error.Bad; }
     return;
 }