Task: Introduce nats-sub-registrar codegen templates

Add a nats-sub-registrar codegen profile that produces a <entity>_registrar.hpp/cpp pair for each refdata entity. The hpp declares register_<entity>_handlers(); the cpp includes only that entity's handler header and wires its NATS subjects to the handler instance. The top-level registrar.cpp will delegate to these in the next task.

• nats-sub-registrar entry exists in facet_catalogue.org with two template/output rows (hpp and cpp).
• cpp_nats_registrar.hpp.mustache and cpp_nats_registrar.cpp.mustache exist in library/templates/ and match the org literate sources.
• cpp_nats_registrar_hpp.org and cpp_nats_registrar_cpp.org exist with tangle blocks referencing the mustache files.
• cpp_messaging.org archetypes table updated to include the two new archetypes.
• Template flags no_history (skip history subject) and list_only (skip save/remove/history) documented in the org sources.

1. Add nats-sub-registrar to library/facet_catalogue.org: two rows (hpp under include/, cpp under src/messaging/).
2. Write cpp_nats_registrar.hpp.mustache — include guard + free function declaration; both domain_entity and entity sections.
3. Write cpp_nats_registrar.cpp.mustache — includes + implementation; list_only flag controls save/remove/history; no_history flag skips history subscription; both model-type sections.
4. Write cpp_nats_registrar_hpp.org and cpp_nats_registrar_cpp.org as org literate sources with tangle blocks.
5. Update cpp_messaging.org summary and archetypes table.

• Approach changed at re-apply time. The original plan targeted the monolithic facet_catalogue.org + standalone cpp_nats_registrar_*.org literate sources. Between first attempt and re-apply, codegen migrated to the physical-space graph: facets/archetypes are now ores.*.org docs under library/templates/ (#+type: facet|archetype, #+facet:, #+output:), discovered by physical_space.py. The templates were re-modelled accordingly (see Result).

Added the nats-sub-registrar codegen facet producing a per-entity <entity>_registrar.hpp/cpp pair. The hpp declares register_<entity>_handlers(); the cpp includes only that entity's handler header and wires its NATS subjects to the handler instance, so a compile failure in one entity's handler cannot block the rest of the service.

Re-modelled onto the physical-space graph (current codegen):

• ores.cpp.nats-sub-registrar.org — facet doc (#+type: facet, #+model_types: domain_entity schema, #+facet_group: ores.cpp).
• ores.cpp.nats-sub-registrar.nats_registrar_header.org and …nats_registrar_implementation.org — archetype docs with #+output: paths, #+facet: back-reference, and the mustache tangle blocks.
• generate.py: nats-sub-registrar → {ores.cpp.nats-sub-registrar} added to the _PROFILE_TO_FACETS shim.
• Tangled to cpp_nats_registrar.{hpp,cpp}.mustache via compass build --direct tangle_codegen_templates.
• Inventories regenerated (regenerate_physical_space_inventories.py): facet now listed in ores.cpp.org.
• list_only wires only the list subject; no_history skips the history subscription — both documented in the archetype docs.

The meta-pattern is documented under Entity-composed registrars, whose NATS instance section links to these archetypes.