Task: Collapse the instrument identity into the trade and split the bond security

Give the component two identities where it carries three, and give the database the relationships the schema can hold. This is item M05 and the identity half of M07, and it implements decision D2: the trade id identifies the trade and its instrument, and the security issue is a separate reference datum.

The rule is already written down. doc/knowledge/architecture/trade_modeling.org says that the economic terms belong to one agreement, that the trade and instrument relationship is one to one, and that "the trade id identifies both". doc/knowledge/domain/trade.org names the four entities: the product type, the security issue where one exists, the trade, and the instrument carrying the economic terms.

The code disagrees. instrument_identity_field_group.org declares instrument_id as a key and trade_id as an optional soft back-reference, and 26 models inherit the group. trade adds a second nullable pointer back at the instrument. The declaration carries 91 instrument_id columns and no foreign key targets an instrument table, so the two directions of a one-to-one relationship are two nullable pointers that nothing can make agree.

This re-opens two tasks the sprint 25 story abandoned: trading_model_cleanup task 7B23DB0F, collapse the instrument identity into the trade, and task 0C525D24, split the bond security from the trade. The order is the order the abandoned split task gave: collapse first, because the reverse reshapes the same tables twice.

• instrument_id is gone from instrument_identity_field_group and from every entity that inherits or references it.
• trade.instrument_id is gone, so the two directions of a one-to-one relationship cannot disagree.
• Instrument tables are keyed by trade_id, not null, with a real foreign key to the trade rather than a soft link.
• An instrument with no real trade is a trade of a reserved type, so there is one shape rather than two.
• The bond security split follows: the shared entities are keyed by the ORE reference datum id, which is SecurityId for a bond and ContractName for a bond future; bond_future keeps only what ORE keeps on the trade; bond_leg and its rate, amount and amortization children hang off the security; bond_issue gains the fields ORE has and we lack, and loses bond_notional, which ORE holds on the trade; and the fields duplicated against the leg tables are held once.
• Defect 7 of the trade model investigation closes: the schema declares the relationships it enforces, instead of 866 check constraints and no foreign keys.
• Defect 9 closes or is recorded: five equity tables hold two products each behind a trade_type_code discriminator.
• Defect 10 closes or is recorded: product_type stops being a database enum on the trades table and becomes the modelled entity.
• Defect 13 closes: one key name for an instrument's identity.
• A trade that states its bond inline rather than by reference still round-trips, so the ORE round trip stays at zero loss across the bond products and the trade envelope.
• check_component_drift.py --component trading-cpp --dry-run reports nothing, and the four suites pass.

• Two documents already describe this change and both are stale in detail only. Read trade_modeling.org for the rule and the abandoned task bodies 7B23DB0F and 0C525D24 for the plan; both are under doc/agile/versions/v0/sprint_25/data-oriented-trading-model/.
• The design's ER page names ores_trading_schedules_tbl while the code ships instrument_schedules. Reconcile the names in the same pass, or record the difference.
• The design's instrument_underlyings table was never built. PR 5 owns that.
• trade_classification.hpp cites ores_refdata_instruments_tbl and trade_identity_field_group.json, and neither exists. Correct the comments here.
• The bond family's facts and legs are still per trade, so the security split is the largest part of the diff. If it cannot land in one PR with the collapse, stop after the collapse with the tree green and open a follow-up rather than leaving the component mid-reshape.