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/event.sx

@@ -52,7 +52,7 @@ Event :: struct {
 Loop :: struct {
     kq: i32 = -1;
 
-    init :: () -> (Loop, !EventErr) {
+    init :: () -> Loop !EventErr {
         q := kqb.kqueue();
         if q < 0 { raise error.Init; }
         return Loop.{ kq = q };
@@ -96,7 +96,7 @@ Loop :: struct {
 
     // Fill `out` with ready events, waiting at most `timeout_ms`
     // (negative = forever). Returns the count; 0 is a timeout.
-    wait :: (self: *Loop, out: []Event, timeout_ms: i64) -> (i64, !EventErr) {
+    wait :: (self: *Loop, out: []Event, timeout_ms: i64) -> i64 !EventErr {
         raw : [64]kqb.Kevent = ---;
         cap : i64 = 64;
         if xx out.len < cap { cap = xx out.len; }