cgen: route aggregate field-to-field assignment through the aggregate copier

The direct-field assignment arms enumerate CALL, STRUCTLIT, and local
IDENT producers; an addressable N_DOT/N_INDEX/deref rhs fell through to
the scalar tail, so a 16-byte struct field copied only its first word.
Resolve both places through the existing address funnels and use the
tail-aware aggregate copier. Both stages.

test/lang/aggregate_field_copy_test.ww

@@ -0,0 +1,73 @@
+// Addressable aggregate fields must copy every byte. A scalar fallback used
+// to copy only the first word of a 16-byte nested value under both stages.
+
+package aggregate_field_copy_test;
+
+type pair = struct {
+	first: i64,
+	second: i64,
+};
+
+type outer = struct {
+	guard: i64,
+	value: pair,
+	tail: i64,
+};
+
+type nested = struct {
+	outer: outer,
+};
+
+let globalsource: outer;
+let globaltarget: outer;
+
+fn check(v: *outer, first: i64, second: i64) void = {
+	assert(v.value.first == first);
+	assert(v.value.second == second);
+	assert(v.guard == 91);
+	assert(v.tail == 92);
+};
+
+@test fn aggregate_field_memory_copy() void = {
+	let source: outer;
+	source.value.first = 11;
+	source.value.second = 12;
+	let target: outer;
+	target.guard = 91;
+	target.value.first = 1;
+	target.value.second = 2;
+	target.tail = 92;
+	target.value = source.value;
+	check(&target, 11, 12);
+
+	let pointer: *outer = ⌖
+	source.value.first = 21;
+	source.value.second = 22;
+	pointer.value = source.value;
+	check(&target, 21, 22);
+
+	let deep: nested;
+	deep.outer.value.first = 31;
+	deep.outer.value.second = 32;
+	target.value = deep.outer.value;
+	check(&target, 31, 32);
+
+	let indexed: [2]pair = [pair{first=5,second=6},
+		pair{first=35,second=36}];
+	target.value = indexed[1];
+	check(&target, 35, 36);
+
+	let pointed: pair = pair{first=37,second=38};
+	let sourcepointer: *pair = &pointed;
+	target.value = *sourcepointer;
+	check(&target, 37, 38);
+
+	globalsource.value.first = 41;
+	globalsource.value.second = 42;
+	globaltarget.guard = 91;
+	globaltarget.value.first = 3;
+	globaltarget.value.second = 4;
+	globaltarget.tail = 92;
+	globaltarget.value = globalsource.value;
+	check(&globaltarget, 41, 42);
+};