Task: Write counterparty documentation (manual chapter, NATS reference)

Add a counterparty chapter (or section) to the user manual covering Qt UI usage, shell/CLI commands, and the counterparty_identifier cardinality-limit and soft-FK validations, following the structure used for other commissioned entities.

• Manual chapter/section documents Qt UI, shell/CLI usage for counterparty.
• Validation rules (soft-FKs, nullability, cardinality limit) are documented for both counterparty and counterparty_identifier.
• Manual PDF rebuilds cleanly with the new content.

(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.)