feat: tuple syntax cutover — Tuple(...) type + .(...) value

Replace the bare-paren tuple grammar with explicit, position-unambiguous
forms, mirroring how structs work:

  type     `(A, B)`        -> `Tuple(A, B)`          (named keeps `:`)
  value    `(a, b)`        -> `.(a, b)`              (named uses `=`)
  typed    (new)           -> `Tuple(A, B).(a, b)`   (like `Point.{...}`)
  failable `-> (T, !)`     -> `-> T !`
           `-> (T1, T2, !)`-> `-> Tuple(T1, T2) !`   (channel outside Tuple)

Bare `(...)` is now grouping only, everywhere; a comma in bare parens is a
hard error with a migration hint. Grouping, function types `(A, B) -> R`,
param lists, lambdas, and match bindings are unaffected.

`Tuple(...)` is strictly a TYPE in every position (including `size_of` /
`type_info` args); a tuple VALUE comes only from `.(...)` (anonymous) or
`Tuple(...).(...)` (explicitly typed). A bare `Tuple(1, 2)` is a tuple
type with non-type elements -> rejected.

The ~110 tuple-bearing corpus files were migrated with a one-shot
AST-aware migrator (the `sx migrate` tool from the prior commit, removed
here). New examples: 0130 (new syntax), 0131 (typed construction), 1060
(named-tuple failable return). 1116 golden updated for the new hint text.

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

@@ -10,7 +10,7 @@
 All :: error { Negative, Other }
 
 // `h` is a bare-`!` Closure slot; the caller declares the union as `!All`.
-dispatch :: (h: Closure(i32) -> (i32, !), x: i32) -> (i32, !All) {
+dispatch :: (h: Closure(i32) -> i32 !, x: i32) -> i32 !All {
     return try h(x);
 }
 
@@ -19,9 +19,9 @@ main :: () -> i32 {
     push Context.{ allocator = xx gpa } {
         // Two literals of the SAME shape raising DIFFERENT tags both feed the
         // one shared `Closure(i32)->(i32,!)` union node.
-        handlers : List(Closure(i32) -> (i32, !)) = .{};
-        handlers.append(closure((x: i32) -> (i32, !) { if x < 0 { raise error.Negative; } return x * 2; }));
-        handlers.append(closure((x: i32) -> (i32, !) { if x == 0 { raise error.Other; } return x + 100; }));
+        handlers : List(Closure(i32) -> i32 !) = .{};
+        handlers.append(closure((x: i32) -> i32 ! { if x < 0 { raise error.Negative; } return x * 2; }));
+        handlers.append(closure((x: i32) -> i32 ! { if x == 0 { raise error.Other; } return x + 100; }));
 
         // success paths
         print("ok0={}\n", dispatch(handlers.items[0], 5) catch (e) 0);   // 10