lang F1 4.2: (..F(Ts)) per-element type application in pack-shaped fields

packTypeElems now handles a parameterized spread operand F(Ts): for each pack
element T_i it temporarily binds the pack name to T_i and resolves F(T_i),
yielding (VL(T0), VL(T1), ...). Combined with parameterized-protocol value
types, the canonical Combined struct field sources: (..VL(Ts)) now resolves to
a tuple of real protocol values.

End-to-end (examples/207): instantiate Combined(s64, s64, string), whole-store
c.sources = (xx IntCell, xx StrCell), and per-element dispatch c.sources.0.get()
/ c.sources.1.get() all work. 242 examples + unit green.

examples/207-combined-pack-field.sx

@@ -0,0 +1,29 @@
+// Phase 4.2 — the canonical `Combined` struct's storage layer: a generic
+// struct whose field is a pack of PARAMETERIZED-protocol values,
+// `sources: (..VL(Ts))` → `(VL(T0), VL(T1), …)`. Each `VL(Ti)` is a real
+// 16-byte protocol value (issue: parameterized-protocol value types), and
+// `(..VL(Ts))` applies `VL` per pack element. Instantiate + whole-tuple store
+// of `xx`-erased values + per-element method dispatch all work.
+
+#import "modules/std.sx";
+
+VL :: protocol(T: Type) { get :: () -> T; }
+IntCell :: struct { v: s64; }
+StrCell :: struct { s: string; }
+impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => 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
+    value:   $R;
+}
+
+main :: () -> s32 {
+    // Combined(s64, s64, string): R=s64, Ts=[s64, string],
+    // sources: (VL(s64), VL(string)).
+    c : Combined(s64, s64, string) = ---;
+    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;
+}