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/packs/0533-packs-pack-tuple-materialize.sx

@@ -14,7 +14,7 @@ impl Box(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }
 impl Box(string) for StrCell { get :: (self: *StrCell) -> string => self.s; }
 
 snapshot :: (..xs: Box) -> void {
-    t := (..xs.get);                 // tuple (i64, string) materialized from the pack
+    t := .(..xs.get);                 // tuple (i64, string) materialized from the pack
     print("0={} 1={}\n", t.0, t.1);
 }
 

examples/packs/0534-packs-pack-type-projection.sx

@@ -20,7 +20,7 @@ impl Box(i64) for Dbl { get :: (self: *Dbl) -> i64 => self.n * 2; }
 // Tuple type `(..xs.T)` — heterogeneous (i64, string), matched by the
 // value-projection `(..xs.get)`.
 snap :: (..xs: Box) -> void {
-    t : (..xs.T) = (..xs.get);
+    t : Tuple(..xs.T) = .(..xs.get);
     print("0={} 1={}\n", t.0, t.1);
 }
 

examples/packs/0538-packs-generic-struct-pack-field.sx

@@ -7,19 +7,19 @@
 
 Box :: struct($R: Type, ..$Ts: []Type) {
     r:    $R;
-    pair: (..$Ts);   // tuple of the pack's element types
+    pair: Tuple(..$Ts);   // tuple of the pack's element types
 }
 
 main :: () -> i32 {
     // Box(i64, i32, string): R=i64, Ts=[i32, string], pair: (i32, string).
     a : Box(i64, i32, string) = ---;
     a.r = 7;
-    a.pair = (42, "hi");                 // whole-tuple field store
+    a.pair = .(42, "hi");                 // whole-tuple field store
     print("a: r={} 0={} 1={}\n", a.r, a.pair.0, a.pair.1);
 
     // A different shape → a different per-position tuple field.
     b : Box(bool, string, bool) = ---;   // Ts=[string, bool], pair: (string, bool)
-    b.pair = ("x", true);
+    b.pair = .("x", true);
     print("b: 0={} 1={}\n", b.pair.0, b.pair.1);
     0
 }

examples/packs/0539-packs-combined-pack-field.sx

@@ -14,7 +14,7 @@ impl VL(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }
 impl VL(string) for StrCell { get :: (self: *StrCell) -> string => self.s; }
 
 Combined :: struct($R: Type, ..$Ts: []Type) {
-    sources: (..VL(Ts));   // (VL(T0), VL(T1), …) — tuple of protocol values
+    sources: Tuple(..VL(Ts));   // (VL(T0), VL(T1), …) — tuple of protocol values
     value:   $R;
 }
 
@@ -22,7 +22,7 @@ main :: () -> i32 {
     // Combined(i64, i64, string): R=i64, Ts=[i64, string],
     // sources: (VL(i64), VL(string)).
     c : Combined(i64, i64, string) = ---;
-    c.sources = (xx IntCell.{ v = 10 }, xx StrCell.{ s = "hi" });
+    c.sources = .(xx IntCell.{ v = 10 }, xx StrCell.{ s = "hi" });
     c.value = 99;
     print("{} {} {}\n", c.sources.0.get(), c.sources.1.get(), c.value);  // 10 hi 99
     0

examples/packs/0540-packs-pack-type-arg-spread.sx

@@ -11,7 +11,7 @@ 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;
 }
 

examples/packs/0541-packs-pack-to-protocol-tuple.sx

@@ -12,13 +12,13 @@ impl VL(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }
 impl VL(string) for StrCell { get :: (self: *StrCell) -> string => self.s; }
 
 Combined :: struct($R: Type, ..$Ts: []Type) {
-    sources: (..VL(Ts));
+    sources: Tuple(..VL(Ts));
     value:   $R;
 }
 
 build :: (..sources: VL) -> void {
     c : Combined(i64, ..sources.T) = ---;
-    c.sources = (..sources);                          // pack → tuple, per-element erase
+    c.sources = .(..sources);                          // pack → tuple, per-element erase
     print("{} {}\n", c.sources.0.get(), c.sources.1.get());
 }
 

examples/packs/0543-packs-canonical-map.sx

@@ -17,14 +17,14 @@ 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; }
 
 map :: (mapper: Closure(..sources.T) -> $R, ..sources: VL) -> VL($R) {
     c : Combined($R, ..sources.T) = ---;
-    c.sources = (..sources);
+    c.sources = .(..sources);
     c.value = mapper(..sources.get);
     return xx c;
 }