docs: write down when a live turn is dropped

AGENTS.md asks for the documentation to move with a runtime-behaviour change, and docs/CONTRACTS.md points at the stable-assistant-turn-anchors RFC for anything that reconciles a live turn with the settled transcript. That is what this change adds, so the rule belongs there.

A new part of the Settlement section says why a preserved live node is not evidence of a running turn, the four facts that have to hold before one is dropped, that ownership is proved from state rather than by comparing two rendered trees, and that only the branches which ADD a turn are gated. It also records the settled-scene half: a prose row that duplicates the final answer, exactly or as a prefix within ten percent of its length, is dropped where both settled renderers take their rows, and live rendering is untouched.

docs/rfcs/stable-assistant-turn-anchors.md

@@ -574,6 +574,38 @@ early renderers can avoid chasing `S.messages` during every paint. If settlement
 later rewrites the transcript message, the anchor must be refreshed from that
 message instead of allowing the two copies to drift silently.
 
+### Settled ownership of a restored live turn
+
+A preserved or snapshotted live-turn node is not evidence of a running turn. The
+settled assistant message is persisted a few milliseconds before the stream's
+terminal event clears the active stream id, so a live node whose stream has
+already ended can survive a transcript rebuild or a session switch and be
+appended under the settled answer, putting the same answer on screen twice
+(#6948 follow-up; upstream symptom report #2051).
+
+A live-turn node is therefore discarded only where the settled transcript is
+proved to own it: the transcript ends with a settled assistant message, no
+message still carries a live-projection marker, that settled message's
+persisted stream identity equals the id of the stream that built the live node
+— or, only where no scene identity was persisted, the markdown the stream
+produced equals the persisted message source — and the node carries nothing the
+settled rebuild could not have produced, meaning no unpersisted tool card,
+reasoning row or transparent-stream row and no second live segment.
+
+Ownership is a state proof, never a comparison of two rendered DOM trees: the
+live body comes from the streaming parser and the settled body from the
+markdown renderer, and an answer that reads the same as the previous one must
+not delete a genuinely live turn. An unrecognised node shape keeps the turn.
+Only the two branches that ADD a turn are gated; a branch that replaces a node
+cannot duplicate one.
+
+A settled activity scene projects one answer as well. A `process_prose` row
+whose text duplicates the scene's final answer — the same text, or a prefix of
+it within ten percent of its length — is dropped where both settled renderers
+take their rows, so the answer is not rebuilt as a second assistant segment
+above itself. Live rendering is unchanged: while the turn streams, the inline
+live segment is hidden and the prose row is the visible answer.
+
 ## Replay, Reload, And Reconstruction
 
 Replay/reload should reconstruct the same Assistant Turn Anchor from durable