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.

library/modules/std/io.sx

@@ -113,7 +113,7 @@ async :: ufcs (io: Io, worker: Closure(..$args) -> $R, ..$args) -> Future($R) {
 
 // `await(f)` — value-carrying failable. `.ready` → the result; `.failed`
 // / `.canceled` → raise the stored / cancellation error.
-await :: ufcs (f: *Future($R)) -> ($R, !IoErr) {
+await :: ufcs (f: *Future($R)) -> $R !IoErr {
     if f.canceled.load(.acquire) { raise error.Canceled; }
     if f.state == .canceled      { raise error.Canceled; }
     if f.state == .failed        { raise error.Failed; }