lang 2.3: tuple materialization from a pack — (..xs) / (..xs.method)

A `spread_expr` element inside a tuple literal now expands the pack into the
tuple's fields: `(..xs.get)` ≈ `(xs[0].get(), …, xs[N-1].get())` (Decision 2 —
a pack is stored by materializing a tuple). lowerTupleLiteral detects a
pack-spread element via packSpreadRefs and splices the per-element Refs as
fields (typed via getRefType); for Box(T) the materialized tuple is
heterogeneous. A spread whose operand isn't a pack falls through to the
existing spread_expr diagnostic (tuple-value spread not yet handled).

When any element is a spread, field-count ≠ element-count, so the contextual
target-tuple alignment is skipped (field types inferred from the expanded refs).
examples/198-pack-tuple-materialize.sx.

examples/198-pack-tuple-materialize.sx

@@ -0,0 +1,25 @@
+// Feature 1 — materialize a tuple from a pack via `(..xs.method)` (Decision 2:
+// a pack is stored by materializing a tuple). `(..xs.get)` projects `get` over
+// the pack and collects the results into a real tuple value, which can then be
+// stored, indexed, and (for `Box(T)`) is heterogeneous per position.
+
+#import "modules/std.sx";
+
+Box :: protocol(T: Type) {
+    get :: () -> T;
+}
+IntCell :: struct { v: s64; }
+StrCell :: struct { s: string; }
+impl Box(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+impl Box(string) for StrCell { get :: (self: *StrCell) -> string => self.s; }
+
+snapshot :: (..xs: Box) -> void {
+    t := (..xs.get);                 // tuple (s64, string) materialized from the pack
+    print("0={} 1={}\n", t.0, t.1);
+}
+
+main :: () -> s32 {
+    snapshot(IntCell.{ v = 42 }, StrCell.{ s = "hi" });
+    snapshot(StrCell.{ s = "x" }, IntCell.{ v = 7 });   // order swapped → (string, s64)
+    0;
+}