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/case.ww

@@ -0,0 +1,25 @@
+//ww:run-exit 0
+// #34 close: append() accepts a struct CALL rvalue element — GP
+// register-class receive (24B str+i32), odd sub-8 (3B), and sized
+// tail (4B) shapes, evaluated pre-grow into a per-site scratch.
+package main;
+type gp24 = struct { name: str, id: i32 };
+type odd3 = struct { a: u8, b: u8, c: u8 };
+type tail4 = struct { v: i32 };
+fn mkgp(id: i32) gp24 = { let e: gp24; e.name = "x"; e.id = id; return e; };
+fn mkodd() odd3 = { let e: odd3; e.a = 1u8; e.b = 2u8; e.c = 3u8; return e; };
+fn mktail() tail4 = { let e: tail4; e.v = 5; return e; };
+fn main() i32 = {
+	let gs: []gp24 = [];
+	append(gs, mkgp(7));
+	append(gs, mkgp(9));
+	if (gs.len != 2 || gs[0].id != 7 || gs[1].id != 9) { return 1; };
+	if (gs[0].name.len != 1) { return 2; };
+	let os_: []odd3 = [];
+	append(os_, mkodd());
+	if (os_[0].a != 1u8 || os_[0].c != 3u8) { return 3; };
+	let ts: []tail4 = [];
+	append(ts, mktail());
+	if (ts[0].v != 5) { return 4; };
+	return 0;
+};