An overlay renews its lease right before Go, then captures and uploads
the element before its generate event leaves. When that outlasts the
lease and its result post is lost, a rescuer can claim the target and
Go, and the helper accepted both generate events: two sessions for one
request.
The generate envelope now carries this page's clientId, and the helper
refuses a generate event for a target that another page holds under a
live lease, or that was already answered with a different session
(`served_agent_targets`, recorded on every ok resolution): 409
`agent_target_already_served`, nothing journaled. The overlay treats
that refusal like a foreign session and hands the surface back. The
answering session's own event stays welcome, so the common path (result
post first, then the event) is unchanged.
Tests: two Rust integration cases (rival lease, answered elsewhere,
welcome for the serving session) and a Node protocol case, contract
pins for the envelope and the refusal handling, contract doc.
Written with AI assistance (Claude).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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