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/1041-errors-failable-closure-shape-union.sx

@@ -24,13 +24,13 @@ main :: () -> s32 {
         handlers.append(closure((x: s32) -> (s32, !) { if x == 0 { raise error.Other; } return x + 100; }));
 
         // success paths
-        print("ok0={}\n", dispatch(handlers.items[0], 5) catch e 0);   // 10
-        print("ok1={}\n", dispatch(handlers.items[1], 7) catch e 0);   // 107
+        print("ok0={}\n", dispatch(handlers.items[0], 5) catch (e) 0);   // 10
+        print("ok1={}\n", dispatch(handlers.items[1], 7) catch (e) 0);   // 107
 
         // failure paths: each closure raises its own tag, which propagates
         // through `try` and is absorbed by the call-site `catch` fallback
-        print("err0={}\n", dispatch(handlers.items[0], -1) catch e -1); // raised Negative → -1
-        print("err1={}\n", dispatch(handlers.items[1], 0) catch e -2);  // raised Other → -2
+        print("err0={}\n", dispatch(handlers.items[0], -1) catch (e) -1); // raised Negative → -1
+        print("err1={}\n", dispatch(handlers.items[1], 0) catch (e) -2);  // raised Other → -2
     }
     return 0;
 }