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/1060-errors-named-tuple-failable.sx

@@ -0,0 +1,26 @@
+// A failable function returning a NAMED tuple value `-> Tuple(x: A, y: B) !E`
+// flattens its value fields into the result tuple (`{ x: A, y: B, err }`),
+// keeping the `.x`/`.y` names addressable on both the success value and the
+// `or` fallback. Exercises the success path, a `raise` path, and an
+// `or .(x=.., y=..)` terminator.
+//
+// Regression (issue 0179-adjacent named-tuple-failable miscompile): a named
+// failable tuple used to WRAP as `{ {A,B}, err }` while the value-return
+// lowering inserted the value slots FLAT, producing invalid LLVM
+// (`Invalid InsertValueInst operands`).
+#import "modules/std.sx";
+
+E :: error { Bad }
+
+two :: (n: i64) -> Tuple(x: i64, y: i64) !E {
+    if n < 0 { raise error.Bad; }
+    return .(x = n, y = n + 1);
+}
+
+main :: () {
+    ok := two(5) or .(x = 0, y = 0);
+    print("{} {}\n", ok.x, ok.y);
+
+    bad := two(-1) or .(x = 0, y = 0);
+    print("{} {}\n", bad.x, bad.y);
+}