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/expected/1010-errors-catch-rejections.stderr

@@ -1,5 +1,5 @@
 error: `catch` requires a failable expression; operand has type 's32'
-  --> /Users/agra/projects/sx/examples/1010-errors-catch-rejections.sx:11:5
+  --> examples/1010-errors-catch-rejections.sx:11:5
    |
-11 |     plain() catch e { return 1; };   // error: operand has type s32 (not failable)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+11 |     plain() catch (e) { return 1; };   // error: operand has type s32 (not failable)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

examples/expected/1013-errors-value-failable-reject.stderr

@@ -1,5 +1,5 @@
 error: `catch` body must produce a value of type 's32' (or diverge with `return` / `raise`)
-  --> /Users/agra/projects/sx/examples/1013-errors-value-failable-reject.sx:17:10
+  --> examples/1013-errors-value-failable-reject.sx:17:10
    |
-17 |     x := parse(-1) catch e { print("oops\n") };   // error: body yields no value
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+17 |     x := parse(-1) catch (e) { print("oops\n") };   // error: body yields no value
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

examples/expected/1037-errors-comptime-run-escape.stderr

@@ -1,4 +1,4 @@
 error: comptime `#run` (x) raised an unhandled error: error.Bad
 error return trace (most recent call last):
   parse at 1037-errors-comptime-run-escape.sx:11:17
-help: handle it at the `#run` site — `#run <expr> catch e { ... }` or `#run <expr> or <default>`
+help: handle it at the `#run` site — `#run <expr> catch (e) { ... }` or `#run <expr> or <default>`

examples/expected/1123-diagnostics-reserved-name-catch-onfail.stderr

@@ -1,11 +1,11 @@
 error: 's64' is a reserved type name and cannot be used as an identifier
-  --> examples/1123-diagnostics-reserved-name-catch-onfail.sx:20:12
+  --> examples/1123-diagnostics-reserved-name-catch-onfail.sx:20:13
    |
-20 |     onfail s64 { }                  // onfail tag binding
-   |            ^^^
+20 |     onfail (s64) { }                  // onfail tag binding
+   |             ^^^
 
 error: 'u8' is a reserved type name and cannot be used as an identifier
-  --> examples/1123-diagnostics-reserved-name-catch-onfail.sx:21:19
+  --> examples/1123-diagnostics-reserved-name-catch-onfail.sx:21:20
    |
-21 |     must(n) catch u8 { return; };   // catch tag binding
-   |                   ^^
+21 |     must(n) catch (u8) { return; };   // catch tag binding
+   |                    ^^

examples/expected/1157-diagnostics-catch-binding-needs-parens.exit

@@ -0,0 +1 @@
+1

examples/expected/1157-diagnostics-catch-binding-needs-parens.stderr

@@ -0,0 +1,5 @@
+error: the catch error binding needs parens: `catch (e) { ... }`
+  --> examples/1157-diagnostics-catch-binding-needs-parens.sx:12:20
+   |
+12 |     v := f() catch e { 0 };
+   |                    ^

examples/expected/1157-diagnostics-catch-binding-needs-parens.stdout

@@ -0,0 +1 @@
+