w6c: append() accepts struct CALL rvalue elements (#34)

Evaluate the call pre-grow into a per-site scratch, receiving by the
N_LET matrix (sret / float / odd-tail / GP), then grow, slot, sized
ladder. Both stages, byte-identical. The two reject pins graduate to
16B GP accept rows; three new fixtures cover 24B GP, 3B/4B tails,
16B float, and 40B sret.

test/wcc/data/r34_append_struct_call_sret/case.ww

@@ -0,0 +1,20 @@
+//ww:run-exit 0
+// #34 close, sret leg: a >24B struct CALL element writes through the
+// hidden RDI into the append scratch before the grow.
+package main;
+type big40 = struct { a: i64, b: i64, c: i64, d: i64, e: i64 };
+fn mkbig(base: i64) big40 = {
+	let e: big40;
+	e.a = base; e.b = base + 1i64; e.c = base + 2i64;
+	e.d = base + 3i64; e.e = base + 4i64;
+	return e;
+};
+fn main() i32 = {
+	let bs: []big40 = [];
+	append(bs, mkbig(10i64));
+	append(bs, mkbig(20i64));
+	if (bs.len != 2) { return 1; };
+	if (bs[0].a != 10i64 || bs[0].e != 14i64) { return 2; };
+	if (bs[1].a != 20i64 || bs[1].e != 24i64) { return 3; };
+	return 0;
+};