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/socket.sx

@@ -93,7 +93,7 @@ SockErr :: error {
 // Accept one pending connection on a nonblocking listener. A connection
 // that died between queueing and accept (ECONNABORTED) is skipped, not
 // surfaced — the listener is fine.
-accept_nb :: (fd: i32) -> (i32, !SockErr) {
+accept_nb :: (fd: i32) -> i32 !SockErr {
     while true {
         c := accept(fd, null, null);
         if c >= 0 { return c; }
@@ -107,7 +107,7 @@ accept_nb :: (fd: i32) -> (i32, !SockErr) {
 
 // Read up to `cap` bytes. Returns the byte count (> 0); an orderly EOF
 // or a peer reset is Closed.
-read_nb :: (fd: i32, buf: [*]u8, cap: usize) -> (i64, !SockErr) {
+read_nb :: (fd: i32, buf: [*]u8, cap: usize) -> i64 !SockErr {
     while true {
         n := read(fd, buf, cap);
         if n > 0 { return xx n; }
@@ -123,7 +123,7 @@ read_nb :: (fd: i32, buf: [*]u8, cap: usize) -> (i64, !SockErr) {
 
 // Write up to `len` bytes, returning how many the kernel took (possibly
 // fewer — the caller continues from there on the next writability).
-write_nb :: (fd: i32, buf: [*]u8, len: usize) -> (i64, !SockErr) {
+write_nb :: (fd: i32, buf: [*]u8, len: usize) -> i64 !SockErr {
     while true {
         n := write(fd, buf, len);
         if n >= 0 { return xx n; }