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/1028-errors-failable-or-chain.sx

@@ -30,8 +30,8 @@ main :: () -> (s32, !E) {
     r = r + (try fa(0) or try fa(7));               // a fails → b succeeds → 7
     r = r + (try fa(0) or try fa(0) or try fa(3));   // first two fail → third → +3  = 10
     r = r + (fa(0) or fa(0) or 96);                  // bare chain + value terminator → +96 = 106
-    r = r + ((try fa(0) or try fa(0)) catch e 5);    // both fail → catch handler → +5  = 111
-    r = r + ((try fa(0) or try fa(9)) catch e 0);    // second succeeds → catch skipped → +9 = 120
+    r = r + ((try fa(0) or try fa(0)) catch (e) 5);    // both fail → catch handler → +5  = 111
+    r = r + ((try fa(0) or try fa(9)) catch (e) 0);    // second succeeds → catch skipped → +9 = 120
 
     try fv(0) or try fv(1);                          // void chain: first fails → second succeeds