Get the conversation-scoped Evidence Trail v2 for a Chat with Customers conversation.

Returns the cached trail when present, otherwise lazy-builds it using the persona on the conversation document. Lazy-build does NOT have access to the original upstream context / mentioned products — those are only available on the chat-stream call path. Lazy-built trails surface only the persona's Gold-grounded market intelligence; connected_nodes and product_context stay empty until the chat stream rebuilds the trail on a subsequent message.