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/1056-errors-enum-value-failable-tuple-and-comptime.sx

@@ -22,13 +22,13 @@ Color :: enum { red; green; blue; }
 E :: error { Nope }
 
 // F1: bare-value success path AND explicit-tuple error path in one function.
-classify :: (s: string) -> (Color, !E) {
+classify :: (s: string) -> Color !E {
     if s == "ok" { return .blue; }        // bare value → {2, 0}
-    return (.red, error.Nope);            // explicit full tuple → {0, 1}
+    return .(.red, error.Nope);            // explicit full tuple → {0, 1}
 }
 
 // F2: comptime parameter forces inline lowering of the body.
-ct_pick :: ($n: i32, s: string) -> (Color, !E) {
+ct_pick :: ($n: i32, s: string) -> Color !E {
     if s == "red"  { return .red; }       // bare value, inline path → {0, 0}
     if s == "blue" { return .blue; }      // bare value, inline path → {2, 0}
     raise error.Nope;                     // inline error path → {undef, 1}