Task: Wire NATS eventing templates into codegen and generate DB-write-to-NATS integration tests

Originally scoped as "wire the NATS eventing templates into codegen" — investigation found that facet (ores.cpp.nats-eventing / ores.cpp.nats-event-registrar) already exists, is already wired into the physical-space graph's default "supported set", and is already proven live (purpose_type, PR #1451/#1459; every other refdata entity too). The real gap: the 6 already-converted trading instrument entities were never generated through the graph's true default set (whatever command was actually used for their conversion, it wasn't the documented =–address ores.cpp=/no-address form), so they're silently missing eventing, request/response messaging, and presentation-history facets the resolver says they're entitled to. Revised goal, three parts, in order:

1. Regenerate the 6 already-converted trading instrument entities through the true default "supported set" (no --address restriction), closing the gap. Eventing and presentation are clean additive gaps; request/response messaging collides with a pre-existing hand-crafted save_* path per entity (same struct, same subject, same service call) — delete the hand-crafted duplicate and adopt the generated registrar instead.
2. Fix the ores.cpp.presentation.history_field_mapper template, the one facet that never learned the has_identity_group=/ =has_audit_group nested-struct branching every other facet (domain, mapper, repository, service) already has — bring it to parity rather than special-case trading.
3. Add a new test facet that generates the "write entity, observe its NATS entity-changed notification" integration test proven by hand in business_unit_eventing_integration_tests.cpp, so every eventing-enabled entity gets this regression coverage for free.

Explicitly out of scope for this task (filed separately once this is proven): migrating refdata and any other flat-generated component to the nested identity=/=audit shape, so the flat branches in every facet template can eventually be deleted and the whole codebase converges on one entity shape.

• All 6 already-converted trading instrument entities (equity_position_instrument, equity_variance_swap_instrument, fx_forward_instrument, equity_forward_instrument, fx_accumulator_instrument, fx_vanilla_option_instrument) regenerated through the true default supported set; every changed file diffed per standing policy, no orphaned hand-crafted files left behind.
• Each entity's hand-crafted save_<entity>_instrument_request/ response struct pair + handler method + queue-subscribe line deleted and replaced by the generated per-entity registrar (list/save/history/remove).
• ores.trading.core's CMakeLists.txt gains the ores.diff.lib dependency the presentation facet needs (mirroring refdata).
• ores.cpp.presentation.history_field_mapper template fixed to branch on has_identity_group=/=has_audit_group exactly like the mapper/repository/service templates already do — no more conditional deletion of generated output to work around it.
• Each entity's generated eventing registrar wired into ores.trading.service's application.cpp eventing pipeline; each entity's generated request/response registrar wired into the relevant registrar_<asset-class>.cpp.
• Second regen pass on every touched entity produces zero further diff (standard codegen drift check).
• Full build + affected ctest suites (trading.*, ore.service, refdata.* untouched) green before PR.
• New integration-test facet generates a Catch2 test matching the shape of business_unit_eventing_integration_tests.cpp, parameterised per entity, wired into the component's tests component_files.cmake.
• A follow-up task filed (not implemented here) to migrate refdata and any other flat-generated component to the nested identity=/=audit shape.

1. Fix ores.cpp.presentation.history_field_mapper_impl.org (tangles to cpp_history_field_mapper.cpp.mustache): branch primary-key, per-column, and audit-block emission on has_identity_group=/ =has_audit_group — same mechanism cpp_domain_type_mapper.cpp.mustache already uses (v.identity.xxx=/=v.audit.xxx vs flat). Retangle via compass build --direct tangle_codegen_templates, diff the tangled .mustache output.
2. Prove it on equity_position_instrument: restore the presentation facet output (regenerate --address ores.cpp.presentation), confirm it now compiles against the nested domain shape.
3. Finish equity_position_instrument end to end: delete its hand-crafted save_equity_position_instrument_request/response (in instrument_protocol.hpp), delete typed_equity_instrument_handler::save_position + its queue-subscribe line in registrar_equity.cpp, wire the generated registrar in instead; wire the generated event registrar into application.cpp; fix any other hand-crafted consumer of the deleted struct (found: ores.ore/service/.../ore_import_execute_handler.cpp, needs the generated protocol header included instead); add ores.diff.lib to ores.trading.core's CMakeLists.txt; regenerate CMake component file lists; build + test; diff every changed file; commit.
4. Repeat step 3 for the other 5 entities, one at a time (mechanical once the template fix and pattern are proven), each its own commit.
5. Design and add the eventing-integration-test facet (parameterise the proven business_unit_eventing_integration_tests.cpp pattern), generate it for at least one trading entity now that trading has real eventing wired, wire into component_files.cmake.
6. File the refdata-migration follow-up task; do not implement it here.

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

Delivered all three revised-goal parts, plus two discoveries fixed along the way:

• Regenerated all 6 already-converted trading instrument entities (equity_position_instrument, equity_variance_swap_instrument, fx_forward_instrument, equity_forward_instrument, fx_accumulator_instrument, fx_vanilla_option_instrument) through the true default codegen supported set, closing the eventing/messaging/presentation gap; each entity's hand-crafted duplicate save_* struct/handler/subscription deleted in favour of the generated registrar. Every file diffed, zero drift on second regen, one commit per entity.
• Fixed ores.cpp.presentation.history_field_mapper to branch on has_identity_group=/=has_audit_group like every other C1202-aware facet — it was the one holdout still assuming a flat entity shape.
• Added the new ores.cpp.eventing-integration-test facet (opt-in): generates a per-entity Catch2 test proving the DB-write -> pg_notify -> postgres_event_source -> event_bus -> NATS-publish chain end to end, no mocks. Enabled and proven (standalone run against live Postgres + NATS, 3/3 assertions each) on all 6 trading instrument entities.
• Discovered and fixed: none of the three CI platforms (continuous-linux/macos/windows, plus nightly-linux) actually started a NATS server — only installed/downloaded the binary. Was latent until this task's tests needed a live NATS; fixed by starting nats-server with the config compass env configure already generates, on all four affected workflows.
• Filed a follow-up task (Migrate all codegen entities to the nested identity/audit struct shape) to eventually converge every entity (refdata included) on the C1202 shape and drop the flat branches everywhere.

Full build + full ctest suite (71/71) green before raising the PR.