layout: a document tab dragged out floats in a workspace of its own

[foxnne]

Part of #260 (step 5 of plans/WORKSPACES_PLAN.md: floats and tear-off).

What changes

Behind FIZZY_WORKSPACES=1, a document tab can float. It floats inside a workspace of its own, so the new window is a tabbed area of documents holding just that one.

• Torn off over the desktop (where floats are OS windows, macOS today): a tab let go over no window of the app's opens a float there, holding a new workspace with that document. Before, nothing happened: a document could not float.
• Dropped on its own pane's middle: the same, the float opening over the pane. The drop's middle shows the float icon there, as it does for a view over its own place.
• A workspace's only tab carries the workspace (State.Documents.alone, from layout: a document lands in any place as a view would #462): the workspace itself floats, rather than a new one being made round the document and the old one closing behind it. Already alone in a float nobody split, it doesn't float again; the pane takes the drop.
• Its float closes when the workspace empties (its last tab closed or carried out), as any float its last view leaves.
• Closing the float gives its documents to the main window's workspace: the one the person was in, when that is in the main window, else the first there. They go in after its own tabs, in order, and the one the float showed is shown. The emptied workspace then goes. With no workspace in the main window, it goes home as a view, as before.

How: State.Documents gets one more member, fold(id) -> bool, which ViewDrag.sendHome asks before sending a closing float's view home. ViewDrag.floatDocument floats the view a document carries alone (carriedView), or a new one (houseView). A new workspace floats out of the pane its photograph was taken in, at that pane's size.

With the flag off, documents is null, and nothing changes.

SDK impact

• None. State.Documents is the app framework's (app/layout), not the plugin boundary.

Verified

• zig build, zig build test, zig build test-integration, zig build check-web, zig build test-sdk-version, zig build check-tests-reached, scripts/check-fmt.sh.
• New integration test, "float: a document tab dropped on its own pane's middle floats in a view holding it". It covers one tab of two floating in a new view, a lone tab floating its workspace, no second float for one already alone, and closing the float folding instead of going home.
• macOS, a sandbox profile run with FIZZY_WORKSPACES=1, driven through the agent plugin (fizzy_act), each checked by fizzy_screen and a capture:
  • tear-c.txt, one of two tabs, dropped on its own pane's middle: Float 1 opens over the pane in an OS window. It holds a new workspace (Pane 3) with only tear-c.txt, and the pane keeps tt-b.pixi.
  • Closing tear-c.txt in the float: the float closes, and the main window is unchanged.
  • tt-a.pixi, its workspace's only tab, dropped on its own pane's middle: that workspace (Pane 4) floats, and the emptied half of Main closes into the other.
• macOS, with the tear-out fixes since merged into a local build (backend: tear-off works again, and a window keeps its size out and back #591, backend: floats are OS windows again on macOS #592, backend: dvui's Debug window tears off too #594): a tab carried past the window's edge opens its own window, holding a new workspace with just that document (Float 1, Pane 6, tear-d.txt).
• Review: fold copied the workspace's tab list but not the id strings in it, then emptied the workspace, and State.setAssignment frees those strings. It now copies the ids too. The gates pass on the branch merged with current main.
• macOS, still to see with a real mouse: a tab torn off over the desktop (my run was interrupted by a person's input), and a float closed from its OS close button folding into the main workspace (the integration test covers the layout side).
• Windows:
• Linux:
• Web: builds; the flag is never on there.

Follow-ups

• A window from ⌘N (step 8) stays open when it empties. The float needs to remember which kind it is then.

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