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/protocols/0414-protocols-generic-struct-protocol-erase.sx

@@ -14,7 +14,7 @@ IntCell :: struct { v: i64; }
 impl VL(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }
 
 Combined :: struct($R: Type, ..$Ts: []Type) {
-    sources: (..VL(Ts));
+    sources: Tuple(..VL(Ts));
     value:   $R;
 }
 impl VL($R) for Combined($R, ..$Ts) { get :: (self: *Combined) -> $R => self.value; }
@@ -22,7 +22,7 @@ impl VL($R) for Combined($R, ..$Ts) { get :: (self: *Combined) -> $R => self.val
 make :: (..sources: VL) -> VL(i64) {
     c : Combined(i64, ..sources.T) = ---;
     c.value = 99;
-    c.sources = (..sources);
+    c.sources = .(..sources);
     return xx c;                 // Combined__i64_i64 -> VL(i64)
 }