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/1024-errors-trace-buffer.sx

@@ -28,13 +28,13 @@ main :: () -> s32 {
     // cleared when the handler completes (a non-diverging exit), not on entry.
     // So inside the handler the frames are still visible (here: the `raise` in
     // `fail` + the `try fail` propagation in `propagate` = 2 frames)...
-    propagate(-1) catch e {
+    propagate(-1) catch (e) {
         print("in catch: len={}\n", sx_trace_len());   // 2 (handler sees the chain)
     };
     print("after catch: len={}\n", sx_trace_len());     // 0 (absorbed at handler exit)
 
     // A success leaves the buffer empty (nothing pushed).
-    propagate(1) catch e { };
+    propagate(1) catch (e) { };
     print("after success: len={}\n", sx_trace_len());   // 0
     return 0;
 }