lang F1 6: contextually type pack-fn prefix args (mapper lambda)

lowerPackFnCall lowered the runtime prefix args with no target_type, so a
lambda arg (mapper: Closure(...) -> ...) could not infer its param types.
Now set target_type to the param type while lowering each prefix arg. With
the existing value-projection call-arg spread, mapper(..sources.get) works:
the lambda is contextually typed and the projected values spread into the
call. examples/211 ((a,b)=>a+b over two sources -> 42). 246 + unit green.

examples/211-mapper-projection-spread.sx

@@ -0,0 +1,18 @@
+// Phase 6 — `mapper(..sources.value)`: project a method over a pack and spread
+// the results into a closure call. The mapper lambda's params are contextually
+// typed from the `Closure(...)` parameter even though `apply` is a pack-fn.
+
+#import "modules/std.sx";
+
+VL :: protocol(T: Type) { get :: () -> T; }
+IntCell :: struct { v: s64; }
+impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+
+apply :: (mapper: Closure(s64, s64) -> s64, ..sources: VL) -> s64 {
+    return mapper(..sources.get);     // (a, b) => a + b  applied to (s0.get(), s1.get())
+}
+
+main :: () -> s32 {
+    print("{}\n", apply((a, b) => a + b, IntCell.{ v = 40 }, IntCell.{ v = 2 }));  // 42
+    0;
+}