v0.2.0

Editing and save quality-of-life: voxel-edge readability, a proper Save / Save
As flow, non-blocking file I/O, and crash-recovery autosave.

Added

• Voxel-edge overlay (View ▸ Voxel edges, on by default): a light wireframe on
  exposed voxel faces so boundaries read even on flat-shaded faces in shadow,
  where coplanar voxels would otherwise blend into one patch (there is no
  ambient occlusion / light baking).
• Save / Save As for the project: Ctrl+S overwrites the open .demiurg file
  without a dialog once its path is known; Save As picks a new path. The kv6 and
  vxl menu entries are now labelled Export.
• File I/O no longer freezes the window (which the OS would flag as hung and
  offer to kill, losing the model): the open/save dialogs run on a worker
  thread off the event loop, and saves serialize/write on a worker thread
  too, with a "Saving…" spinner.
• Background autosave: while there are unsaved changes the project is snapshotted
  to the OS temp dir every 20 s; on the next launch a surviving autosave (after a
  crash) is loaded automatically with a "Recovered work" banner. A clean exit
  removes it.

Fixed

• Startup could open a white, frozen window on some Windows GPUs/drivers/remote
  sessions: the forced Fifo (vsync) present mode could stall present()
  indefinitely. Present is now uncapped — the ~60 fps frame timer already caps
  GPU load — and --cpu was added as an escape hatch (alongside ROXLAP_GPU=0)
  when GPU device creation itself hangs.
• Place tool: when the cursor ray hits no voxel it now falls back to the model's
  floor (the volume's bottom face), so you can seed voxels — and rebuild a model
  emptied of its last voxel — instead of having nothing to click.