Resolve preview's page-to-node lookup for every layerHEADmaster

pages_controller#preview loaded a page by id and rendered it
directly, assuming @page.node was always present. True for head and
draft, since both carry a real node_id: false for autosave, which
deliberately has node_id: nil (the mechanism that excludes it from
Node#pages and the revision count). A node with an autosave and no
draft underneath it: lock, start editing, never explicitly save a
draft: loads that autosave directly with no node at all, and any
view code assuming @page.node is real (headline image, credits,
gallery links) raises.

When @page has no node, resolve it by checking autosave_id, then
draft_id, then head_id on Node, and set @page.node in memory only:
the autosave's node_id: nil is never persisted differently. The
existing swap from draft to a fresher autosave, when both exist, is
unchanged.

Three tests: draft with a fresher autosave on top, autosave with no
draft underneath, and the ordinary head-only case with neither.

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