w6c+selfhost: cross-module same-leaf type disambiguation via Sym.mod

test/wcc/data/modcollision/mod1/mod1.ww

@@ -0,0 +1,8 @@
+// modcollision/mod1 — exports `type stream` with mod1-shaped fields.
+// Paired with mod2/mod2.ww to exercise same-leaf-name cross-module
+// type disambiguation. Test driver: 696_modtype_leaf_collision.c.
+
+export type stream = struct {
+	a: i32,
+	b: i32,
+};