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/0122-types-flags.sx

@@ -81,26 +81,26 @@ main :: () {
 
     // Basic tuple destructuring
     {
-        da, db := (10, 20);
+        da, db := .(10, 20);
         print("basic: {} {}\n", da, db);
     }
 
     // Destructure from function return
     {
-        dswap :: (a: i64, b: i64) -> (i64, i64) { (b, a) }
+        dswap :: (a: i64, b: i64) -> Tuple(i64, i64) { .(b, a) }
         dx, dy := dswap(1, 2);
         print("fn: {} {}\n", dx, dy);
     }
 
     // Discard with _
     {
-        _, dsecond := (100, 200);
+        _, dsecond := .(100, 200);
         print("discard: {}\n", dsecond);
     }
 
     // Three elements
     {
-        da3, db3, dc3 := (1, 2, 3);
+        da3, db3, dc3 := .(1, 2, 3);
         print("triple: {} {} {}\n", da3, db3, dc3);
     }
 }