Closing a workspace pane does not persist its open tabs (no autosave on close; tabs opened since last Ctrl+S are lost on reopen)

[JoshuaRowePhantom]

Summary

When a workspace pane is closed, its currently-open tabs are not persisted unless the user has explicitly saved (Ctrl+S / the save button). Any tab opened since the last explicit save is therefore not present in the persisted dock-layout, so reopening the workspace does not restore it. The design owner has noted that autosave-on-close is not a current requirement; this issue is split out of #1340 (which covers a separate document-restore failure) so it can be tracked independently and prioritised later.

Root Cause

The pane-close path MainWindowViewModel.CloseWorkspaceCommand → OnCloseWorkspace → CloseWorkspacePaneAsync → RemoveWorkspacePaneAsync (features\Phantom.Workspaces\ViewModels\MainWindowViewModel.cs:2288-2329) disposes the pane but never persists it. The only persistence entrypoint is the explicit save: WorkspacePaneViewModel.SaveCommand → SaveAsync → SaveWorkspacePaneAsync → WriteBackWorkspaceTabs (MainWindowViewModel.cs:2786-2903, 2905-2906); WriteBackWorkspaceTabs has exactly one caller (the explicit save). Because RemoveWorkspacePaneAsync does not call it, tabs opened since the last explicit save are never written to the workspace entity's dock-layout, and are absent on the next reopen.

Affected Files

File	Contribution
features\Phantom.Workspaces\ViewModels\MainWindowViewModel.cs	RemoveWorkspacePaneAsync (:2288-2329) disposes without persisting; WriteBackWorkspaceTabs (:2786-2903) single-caller (explicit save only); SaveWorkspacePaneAsync (:2905-2906)
features\Phantom.Workspaces\ViewModels\WorkspacePaneViewModel.cs	SaveCommand/SaveAsync — the only persistence trigger today

Design / Fix (deferred — not a current requirement)

If/when autosave-on-close is desired, have the pane-close path persist the pane's tabs before disposal — e.g. await WriteBackWorkspaceTabs(pane) (or the post-#1341 equivalent on WorkspacePaneViewModel) inside RemoveWorkspacePaneAsync before pane.DisposeAsync(). Consider whether this should be unconditional or gated (dirty-tracking / a setting), since unconditionally writing on every close changes current save semantics. Note: after #1341 relocates per-pane persistence onto WorkspacePaneViewModel, the natural home for this is a pane-level SaveOnCloseAsync.

Expected Tests

Test Name	Class	What It Verifies
RemoveWorkspacePaneAsync_WithUnsavedTabs_PersistsTabsBeforeDisposal	MainWindowIntegrationTests	Closing a pane with unsaved tabs writes them back before disposal (once implemented)
WorkspaceReopen_AfterCloseWithoutExplicitSave_RestoresTabsOpenedSinceLastSave	MainWindowIntegrationTests	Reopen restores tabs that were opened after the last explicit save

[JoshuaRowePhantom]

Split out of #1340. This is the "no autosave on close" concern, which the design owner has stated is not a current requirement — filed separately so #1340 can focus on the document-restore failure.