Task: Add the trade group and its membership

A run's trade group names every trade it read or wrote and the role each played.

• The trade group carries a foreign key to its run, and the membership is keyed by the group and the trade with enforced foreign keys.
• The role comes from the process the run is a run of, not from a global list.
• The roles tell what the run read from what it wrote, and a trade may be in both.
• No row of the membership references a trade activity: the activity is the stamp on an amendment, and the run is the parent here.

(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.)

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