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/types/0128-types-tuples.sx

@@ -9,18 +9,18 @@ main :: () {
     // --- Tuples ---
     {
         print("=== Tuples ===\n");
-        pair := (40, 2);
+        pair := .(40, 2);
         print("{}\n", pair.0);
         print("{}\n", pair.1);
 
-        named := (x: 10, y: 20);
+        named := .(x = 10, y = 20);
         print("{}\n", named.x);
         print("{}\n", named.0);
 
-        single := (42,);
+        single := .(42);
         print("{}\n", single.0);
 
-        zeroed : (i32, i32) = ---;
+        zeroed : Tuple(i32, i32) = ---;
         print("{}\n", zeroed.0);
         print("{}\n", zeroed.1);
     }