Add attached Work Column docking #980

Acceptance Criteria

• An attached Work Column can render on the right, left, or bottom of Chat from one dock action API.
• Right and left reuse one persisted side width; bottom persists an independent height for the same editor tab.
• A newly created editor tab uses the default right dock, while reload restores its saved attached geometry.
• The resize handle appears on the Work Column-facing edge for every dock and clamps values to feasible geometry.
• A dock change preserves the same in-document Preview renderer instance and active inspector state.
• When an editor is too small for a target dock, the action reports infeasibility and leaves the current dock unchanged.
• Resizing never silently detaches or changes the Work Column lease owner.
• Mobile and legacy layouts remain unchanged.

Testing Decisions

Extend the existing Work Column geometry tests with every dock transition, editor-scoped persistence, side and bottom dimensions, infeasible targets, and resize clamping. Extend the Preview browser regression suite to compare stable renderer instance identity across all attached dock transitions.

Key Decisions

• Presentation only -- attached geometry does not transfer, journal, or recreate logical Work Column content.
• One side dimension -- crossing Chat does not create a second width preference; bottom is the sole orientation-specific dimension.
• Feasibility before mutation -- an invalid target refuses before changing layout state.

Constraints & Invariants

• This slice must retain Add durable Work Column record and lease journal #979's attached lease throughout every transition.
• Attached in-document Preview identity remains intact.
• The dock action is intentionally interaction-agnostic so later drag and command UI can use it.

Prior Art

• Existing Work Column width policy and resize tests.
• Existing Preview retained-renderer browser contract.

Source

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

Review Amendment (2026-09-10)

• Key attached presentation by Add durable Work Column record and lease journal #979's Work Column ID, not the current server-global layout value. Persist dock, side width, and bottom height under that identity; migrate the legacy global width and default fresh records to right dock.
• Establish one stable attached Work Column host and move it with grid areas or equivalent CSS only. Branch-specific JSX may not mount a second panel or dispose the existing one.
• Add a browser sentinel that proves stable Work Column mount identity, active Preview file, dirty content, mode, zoom, selected inspector state, and renderer identity through every attached dock transition.
• Define a pure geometry resolver with explicit minimum Chat and Work Column width and height, gap accounting, and discriminated infeasibility results that leave geometry, record revision, and lease unchanged.
• Make the attached host own the only outer resize handle: right uses horizontal start, left horizontal end, and bottom vertical start. Disable competing nested v2 handles.
• Add accessible resize semantics: focusable separator, orientation and value bounds, Arrow/Home/End behavior, visible focus, and browser coverage. Keep command bindings in Add Work Column detach gestures and keyboard commands #985.
• Keep dock preferences inert outside desktop new-layout mode and add mobile and legacy regression coverage.