xo-gc stack: refactor + streamline.

Retiring unused Collector typealiases.
Fix #include topology.
Fix/improve write barrier setup.

codegen/genfacet

@@ -154,11 +154,13 @@ def gen_facet(env,
     # RFoo.hpp
     router_facet_hpp_fname = f'{router_facet}.hpp'
 
+    # REMINDER: this context for FACET definition, e.g. AGCObject
     context = {
         'genfacet': 'xo-facet/codegen/genfacet',
         'genfacet_input': idl_fname,
         'using_dox': using_dox,
-        'impl_hpp_subdir': facet_detail_subdir,
+        'impl_hpp_subdir': facet_detail_subdir,    # legacy name
+        'facet_detail_subdir': facet_detail_subdir,
         #
         'facet_hpp_j2': 'facet.hpp.j2',
         'facet_includes': facet_includes,

codegen/iface_facet_xfer.hpp.j2

@@ -9,10 +9,18 @@
  *       [{{ iface_facet_any_hpp_j2 }}]
  *  3. idl for facet methods
  *       [{{ idl_fname }}]
+ *
+ *  variables:
+ *    {facet_hpp_fname} -> {{facet_hpp_fname}}
+ *    {impl_hpp_subdir} -> {{impl_hpp_subdir}}
+ *    {facet_ns1} -> {{facet_ns1}}
+ *    {facet_detail_subdir} -> {{facet_detail_subdir}}
+ *    {abstract_facet_fname} -> {{abstract_facet_fname}}
  **/
 
 #pragma once
 
+#include "{{abstract_facet_fname}}"
 {% for include_fname in facet_includes %}
 #include {{include_fname}}
 {% endfor %}