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/thread.sx

@@ -106,7 +106,7 @@ Thread :: struct {
 
     // `entry` is the C->sx boundary: abi(.c), fabricates its own
     // Context before touching default-conv sx code (examples/1636).
-    spawn :: (entry: (*void) -> *void abi(.c), arg: *void) -> (Thread, !ThreadErr) {
+    spawn :: (entry: (*void) -> *void abi(.c), arg: *void) -> Thread !ThreadErr {
         t : Thread = .{};
         if pthread_create(@t.handle, null, entry, arg) != 0 { raise error.Spawn; }
         return t;
@@ -144,7 +144,7 @@ Pool :: struct {
 
     // Heap-allocate (the pool must never move: workers hold its address,
     // and it embeds a live mutex), init in place, spawn the workers.
-    create :: (workers: i64, backlog: i64) -> (*Pool, !ThreadErr) {
+    create :: (workers: i64, backlog: i64) -> *Pool !ThreadErr {
         alloc := context.allocator;
         p : *Pool = xx alloc.alloc_bytes(size_of(Pool));
         p.* = Pool.{};