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/diagnostics/expected/1116-diagnostics-tuple-type-nontype-element-rejected.stderr

@@ -1,5 +1,5 @@
-error: tuple type element is not a type (found `int_literal`); a tuple used as a type must list only types, e.g. `(i32, i32)`
-  --> examples/diagnostics/1116-diagnostics-tuple-type-nontype-element-rejected.sx:11:55
+error: tuple type element is not a type (found `int_literal`); a tuple used as a type must list only types, e.g. `Tuple(i32, i32)`
+  --> examples/diagnostics/1116-diagnostics-tuple-type-nontype-element-rejected.sx:11:60
    |
-11 |     print("bad tuple type size = {}\n", size_of((i32, 1)));
-   |                                                       ^
+11 |     print("bad tuple type size = {}\n", size_of(Tuple(i32, 1)));
+   |                                                            ^