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/basic/0036-basic-ufcs-aliases.sx

@@ -28,14 +28,14 @@ main :: () {
         print("{}\n", 1 |> calc(2, 3, 4));     // same = 3 — pipe UFCS
 
         // Tuple return type
-        swap :: (a: i64, b: i64) -> (i64, i64) { (b, a) }
+        swap :: (a: i64, b: i64) -> Tuple(i64, i64) { .(b, a) }
         s := swap(1, 2);
         a := s.0;
         b := s.1;
         print("{}\n", a);                // 2
         print("{}\n", b);                // 1
 
-        wrap :: (x: i64) -> (i64,) { (x,) }   // 1-tuple needs trailing comma; (i64) groups
+        wrap :: (x: i64) -> Tuple(i64) { .(x) }   // 1-tuple needs trailing comma; (i64) groups
         t := wrap(99);
         print("{}\n", t.0);             // 99
     }

examples/basic/0038-basic-dead-code-after-terminator.sx

@@ -17,7 +17,7 @@ E :: error { Neg }
 const_one :: () -> i64 { return 1; return 99; }
 
 // dead `return x;` after an unconditional raise (the failable closure shape)
-always_raise :: (x: i64) -> (i64, !E) { raise error.Neg; return x; }
+always_raise :: (x: i64) -> i64 !E { raise error.Neg; return x; }
 
 // guard: a conditional return must still fall through to the trailing return
 clamp :: (x: i64) -> i64 { if x > 10 { return 10; } return x; }

examples/basic/0042-basic-block-value-destructure.sx

@@ -8,7 +8,7 @@
 
 #import "modules/std.sx";
 
-pair :: () -> (i32, i32) { (5, 7) }
+pair :: () -> Tuple(i32, i32) { .(5, 7) }
 
 main :: () -> i32 {
     // destructure decl inside a value-bound block