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/0038-basic-dead-code-after-terminator.sx

@@ -24,7 +24,7 @@ clamp :: (x: s64) -> s64 { if x > 10 { return 10; } return x; }
 
 main :: () -> s32 {
     print("const_one={}\n", const_one());            // 1
-    print("raised={}\n", always_raise(5) catch e 0); // 0
+    print("raised={}\n", always_raise(5) catch (e) 0); // 0
     print("clamp_hi={}\n", clamp(42));               // 10
     print("clamp_lo={}\n", clamp(7));                // 7