Two gaps in the autosave recovery files:
1. A brand-new, never-saved drawing was never autosaved — on_autosave only
wrote tabs that already had a path. A shared autosave_target() now writes
saved drawings to <file>.sv$ and unsaved ones to the system temp dir
(OpenCADStudio_<name>.sv$).
2. Closing a file (tab) left its .sv$ behind, which also defeated the
program-close cleanup (it only scans still-open tabs). on_tab_close and the
unsaved-discard path now delete the closing tab's .sv$, and Save/Save-As
drop the temp copy a drawing used while it was still unsaved.
Program-close cleanup (exit_app -> cleanup_autosaves) already worked.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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