Add full Preview state codec for Work Column transfer #981

Acceptance Criteria

• A Preview checkpoint round-trips pane tree, stable pane IDs, tab order, active tab, focused leaf, zoom, and canvas scroll.
• Every open document round-trips resource identity, base revision, dirty content, mode, multi-selection, renderer scroll anchor, undo/redo history, pending save or close operation, save outcome, and conflict state.
• A Preview mutation is write-ahead journaled through Add durable Work Column record and lease journal #979 before it is treated as committed by the host.
• Hydrating a checkpoint recreates equivalent editable state, including undo/redo and dirty-close behavior, without duplicating an open document.
• A stale save or close result whose operation ID or lease generation no longer matches is ignored without changing current document state.
• A failed codec or checksum validation leaves the current host lease unchanged and reports hydration failure.
• Ordinary in-document Preview moves keep physical renderer identity; cross-webview hydration creates a new physical instance only after logical state validates.

Testing Decisions

Create codec round-trip and operation-order tests for documents, pane trees, dirty drafts, multi-selection, undo/redo, save, close, and conflicts. Extend the existing Preview browser suite with an editable-document parity fixture that compares outcomes before and after hydrate.

Key Decisions

• Logical document parity -- cross-webview transfer preserves document semantics and history, not the original editor object.
• Fenced async results -- save and close completion apply only to the exact operation and lease generation that initiated them.
• Checkpoint validation -- codec version and canonical checksum must agree before a destination may acknowledge hydration.

Constraints & Invariants

• The eight-document limit remains a logical-document limit.
• No dirty draft or acknowledged editor operation is lost across hydrate.
• This slice does not create a detached host; it supplies its required state codec.

Prior Art

• Existing Preview workspace, retained renderer, editor, and dirty-close behavior.
• Existing Preview browser regression coverage.

Source

Part of #977. Blocked by #979. ADR 0014, Dockable Work Column.

Review Amendment (2026-09-10)

• Resolve state ownership with the ADR amendment: the workspace owns the pane tree and split-node geometry plus outer-canvas scroll; each leaf owns tab order, active tab, focus, and zoom; each document owns mode, draft, selections, editor history, and renderer scroll.
• Add conditional file writes: reads expose a content revision; writes require an expected revision and return either the next revision or authoritative conflict content and revision.
• Define a versioned editor-state adapter containing document text, all selection ranges, primary range, history groups, and replay or mapping policy. Hydration constructs the editor from this adapter before accepting input.
• Define document revisions as provisional, journal-acknowledged, and transferable. Checkpoint quiescence drains through a named acknowledged revision with retry and backpressure behavior.
• State that a restored dirty draft is authoritative over its matching base revision. Late reads refresh clean state only or surface a conflict, never overwrite the restored draft.
• Define scroll-anchor variants and restore timing for workspace, text editor, markdown preview, image, and PDF. Explicitly exclude non-serializable visual selections where no logical equivalent exists.
• Define save and close transitions keyed by operation ID, lease generation, and document revision; validate hydrate referential integrity for unique resources, pane IDs, active tabs, focus IDs, split ratios, and the eight-document limit.
• Add bidirectional two-webview tests for undo/redo, multi-selection, delayed reads and saves, close-transfer races, stale completions, invalid checkpoints, document-kind anchors, and ordinary in-document renderer identity.