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.

src/ir/lower/error.zig

@@ -323,12 +323,20 @@ pub fn buildFailableTuple(self: *Lowering, ret_ty: TypeId, value_refs: []const R
 /// dropped) for a multi-value one. Callers must pass a value-carrying
 /// tuple — a pure `-> !`'s success type is `void`, handled separately.
 pub fn failableSuccessType(self: *Lowering, op_ty: TypeId) TypeId {
-    const fields = self.module.types.get(op_ty).tuple.fields;
+    const tup = self.module.types.get(op_ty).tuple;
+    const fields = tup.fields;
     const n_vals = fields.len - 1;
     if (n_vals == 1) return fields[0];
+    // Carry the value-field names through, dropping the trailing error-slot
+    // name, so a named failable tuple `-> Tuple(x: A, y: B) !` yields a value
+    // type `(x: A, y: B)` whose `.x`/`.y` fields stay addressable.
+    const succ_names: ?[]const types.StringId = if (tup.names) |ns|
+        self.alloc.dupe(types.StringId, ns[0..n_vals]) catch unreachable
+    else
+        null;
     return self.module.types.intern(.{ .tuple = .{
         .fields = self.alloc.dupe(TypeId, fields[0..n_vals]) catch unreachable,
-        .names = null,
+        .names = succ_names,
     } });
 }