* Archive v3 docs under /v3 and publish v4 as the primary version
* Label primary docs version v4.0.0 (alpha 1)
* Add What's New in v4 page; fix upgrade-guide phrasing; point banner at What's New
* Rewrite What's New around v4's new capabilities, not the sampling deprecation
* Lead What's New with the SDK v2 engine swap and the SEPs it brings
* State ships now (link Session State); tasks arrive next alpha
* Exclude docs/v3 frozen snapshots from doc-example import validation