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/sched.sx

@@ -804,7 +804,7 @@ go :: ufcs (self: *Scheduler, work: Closure() -> $R) -> *Task($R) {
 // Suspend the caller until the task completes; return its value (or raise on
 // cancel). MUST be called from inside a fiber (so there is a `self.current` to
 // park) — typically from a fiber spawned via `s.spawn(...)`.
-wait :: ufcs (t: *Task($R)) -> ($R, !TaskErr) {
+wait :: ufcs (t: *Task($R)) -> $R !TaskErr {
     if t.canceled != 0 { raise error.Canceled; }
     if t.state == .pending {
         // ONE waiter per task (enforced). A `Task` holds a single `waiter` slot;