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)
 }
 

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;
 }

examples/protocols/0824-protocols-same-name-method-wrapped-own-wins.sx

@@ -23,27 +23,27 @@ Provider :: protocol {
     // discriminating wrapped/compound RETURNS
     getp :: (self: *Self) -> *Box;
     geto :: (self: *Self) -> ?Box;
-    gett :: (self: *Self) -> (Box, Box);
+    gett :: (self: *Self) -> Tuple(Box, Box);
     geta :: (self: *Self) -> [2]Box;
     // routing-only wrapped/compound PARAMS
     sump :: (self: *Self, p: *Box) -> i32;
     sumo :: (self: *Self, o: ?Box) -> i32;
     sums :: (self: *Self, s: []Box) -> i32;
     suma :: (self: *Self, a: [2]Box) -> i32;
-    sumt :: (self: *Self, t: (Box, Box)) -> i32;
+    sumt :: (self: *Self, t: Tuple(Box, Box)) -> i32;
     sumn :: (self: *Self, n: *?[]Box) -> i32;
 }
 
 impl Provider for Holder {
     getp :: (self: *Holder) -> *Box { @self.b }
     geto :: (self: *Holder) -> ?Box { self.b }
-    gett :: (self: *Holder) -> (Box, Box) { (self.b, self.b) }
+    gett :: (self: *Holder) -> Tuple(Box, Box) { .(self.b, self.b) }
     geta :: (self: *Holder) -> [2]Box { r : [2]Box = ---; r[0] = self.b; r[1] = self.b; r }
     sump :: (self: *Holder, p: *Box) -> i32 { p.m }
     sumo :: (self: *Holder, o: ?Box) -> i32 { o!.m }
     sums :: (self: *Holder, s: []Box) -> i32 { s[0].m }
     suma :: (self: *Holder, a: [2]Box) -> i32 { a[0].m }
-    sumt :: (self: *Holder, t: (Box, Box)) -> i32 { t.0.m }
+    sumt :: (self: *Holder, t: Tuple(Box, Box)) -> i32 { t.0.m }
     sumn :: (self: *Holder, n: *?[]Box) -> i32 { if n == null { 0 } else { 6 } }
 }
 
@@ -63,7 +63,7 @@ main :: () -> i32 {
     arr : [2]Box = ---; arr[0].m = 2; arr[1].m = 3;
     sl : []Box = arr[0..2];
     osl : ?[]Box = sl;
-    tup : (Box, Box) = (one, one);
+    tup : Tuple(Box, Box) = .(one, one);
 
     sp := p.sump(@one);
     so := p.sumo(one);