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_float/case.ww

@@ -0,0 +1,18 @@
+//ww:run-exit 0
+// #34 close, float leg: a float-bearing <=16B struct CALL element is
+// received on independent SSE/GP return cursors (the #171a matrix).
+package main;
+type flt16 = struct { x: f64, n: i64 };
+fn mkflt(v: f64, n: i64) flt16 = {
+	let e: flt16; e.x = v; e.n = n; return e;
+};
+fn main() i32 = {
+	let fs: []flt16 = [];
+	append(fs, mkflt(2.5, 11i64));
+	append(fs, mkflt(0.25, 13i64));
+	if (fs.len != 2) { return 1; };
+	if (fs[0].n != 11i64 || fs[1].n != 13i64) { return 2; };
+	if (fs[0].x != 2.5) { return 3; };
+	if (fs[1].x != 0.25) { return 4; };
+	return 0;
+};