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/platform/1640-platform-asm-parse.sx

@@ -5,7 +5,7 @@
 // `.build`: ir-only on a non-x86 host (the `.ir` snapshot locks the struct
 // return + `%[name]` rewrite); runs natively on x86_64-linux. See 1647 for a
 // multi-output example that executes on aarch64.
-divmod :: (n: u64, d: u64) -> (quot: u64, rem: u64) {
+divmod :: (n: u64, d: u64) -> Tuple(quot: u64, rem: u64) {
     return asm {
         "divq %[d]",
         [quot] "={rax}" -> u64,

examples/platform/1647-platform-asm-aarch64-multi.sx

@@ -3,7 +3,7 @@
 // a `(lo, hi)` tuple. The two outputs become an LLVM `{ i64, i64 }` struct =
 // sx's tuple. aarch64-pinned via `.build`: executes on a matching host (exit
 // reflects lo+hi), ir-only elsewhere.
-split :: (x: u64) -> (lo: u64, hi: u64) {
+split :: (x: u64) -> Tuple(lo: u64, hi: u64) {
     return asm {
         #string ASM
         and %[l], %[x], #0xff