Task: Author the equity family header

The equity family has one header keyed by the trade, carrying the identity, the common dates and the free text, and no economic field.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

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

The equity family has a header. equity_instruments is keyed by the trade and carries only the columns every routed code shares: the identity, start_date and maturity_date as the family's common dates, and the description. It carries no economic field, because none is common to every code, and no underlying, because two products state a basket.

One model declaration generated the domain type, JSON I/O, the table, the entity, the repository, the messaging, the eventing, the shell command with its test, and the SQL create and drop files, with the party and trade-type indexes and a date-order check.

Not done, and named: the routing is not re-pointed. The catalogue's equity codes still name the nine product tables and the importer still writes them, so the header has no coverage check yet. Re-pointing first would leave imports writing rows the catalogue does not name. The routing, the coverage check and the fact tables' duplicated identity columns all move with the mapper migration, which the operator deferred to the xsdcpp removal.

Verification. The trunk was broken when the unit was built, in two independent ways, both from the conventions refactor rather than from this change. The C++ half is fixed by #3474 and the TypeScript half by #3482, both mergeable and now merged, after which this unit's full build passes and the whole suite passes 96 of 96 with the database recreated for the new table. The codegen drift for the trading component regenerates exactly what is committed.