test: port 944_alias_cgen_b5_run to a compile fixture

944_alias_cgen_b5_run.c -> alias_g73_heapfill (//ww:compile), joining
its migrated alias_* siblings. The carrier's stated fixture blocker
('//ww:compile links') is stale: the COMPILE cell runs the bare
frontend, no link. The live #24 field(SB) deref-read leak enters
DATABYTEID_DIVERGED, upgrading the carrier's silent byte-id waiver
with a graduation tripwire.

test/wcc/data/alias_g73_heapfill/case.ww

@@ -0,0 +1,12 @@
+//ww:compile
+// migrated from test/wcc/944_alias_cgen_b5_run.c: heap struct-lit str-arm fill through a 2-level str alias — pins the absence of the pre-c2 cstage #73 fatal; ww .s carries the #24 field(SB) deref-read leak (DATABYTEID_DIVERGED).
+package main;
+type s1t = str;
+type s2t = s1t;
+type box = struct { s: s2t, n: int };
+export fn main() i32 = {
+	let p = alloc(box { s = "hello", n = 5 })!;
+	if (p.n != 5) { return 1; };
+	if (p.s.len != 5) { return 2; };
+	return 0;
+};