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/0415-protocols-protocols.sx

@@ -141,7 +141,7 @@ SmokeErr :: error { Empty, BadDigit, Overflow }
 // value-carrying, named set: raise three tags or succeed
 
 // value-carrying, named set: raise three tags or succeed
-sm_parse :: (n: i32) -> (i32, !SmokeErr) {
+sm_parse :: (n: i32) -> i32 !SmokeErr {
     if n < 0  { raise error.BadDigit; }
     if n == 0 { raise error.Empty; }
     if n > 99 { raise error.Overflow; }
@@ -159,10 +159,10 @@ sm_check :: (ok: bool) -> ! {
 // multi-value, inferred set: `try` propagates; the SCC pass absorbs SmokeErr
 
 // multi-value, inferred set: `try` propagates; the SCC pass absorbs SmokeErr
-sm_pair :: (a: i32, b: i32) -> (i32, i32, !) {
+sm_pair :: (a: i32, b: i32) -> Tuple(i32, i32) ! {
     x := try sm_parse(a);
     y := try sm_parse(b);
-    return (x, y);
+    return .(x, y);
 }
 
 // `catch` block that diverges (logs the tag, then returns a fallback)
@@ -178,7 +178,7 @@ sm_or_default :: (n: i32) -> i32 {
 // `onfail` + `defer` interleave: cleanup runs only on the error path
 
 // `onfail` + `defer` interleave: cleanup runs only on the error path
-sm_acquire :: (fail: bool) -> (i32, !) {
+sm_acquire :: (fail: bool) -> i32 ! {
     defer  print("  smoke defer A\n");
     onfail print("  smoke onfail B\n");
     if fail { raise error.Acquire; }
@@ -188,7 +188,7 @@ sm_acquire :: (fail: bool) -> (i32, !) {
 // `or`-chain: try a, fall to try b; propagate if both fail
 
 // `or`-chain: try a, fall to try b; propagate if both fail
-sm_first :: (a: i32, b: i32) -> (i32, !) {
+sm_first :: (a: i32, b: i32) -> i32 ! {
     v := try sm_parse(a) or try sm_parse(b);
     return v;
 }