TNO Threading (draft):
Each origin feed numbers new threads (tno:N). Replies carry both (tno:N) and (ofeed:<origin-url>). Thread identity = (ofeed, tno).

• Roots: (tno:N) (implicit ofeed=self).
• Replies: (tno:N) (ofeed:<url>).
• Clients: increment tno locally for new threads, copy tags on reply.
• Subjects optional, not required.

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