Keep another process's downloads when opening a library (closes #105)

The download writer's temp files are now named .quak-<pid>-<random>.tmp.
removeLeftoverTempFiles moves from the backup into the download module and
deletes a .quak-*.tmp file only when the process ID in its name is no
longer running; the content cache calls it at open() instead of deleting
every temp file, so a download in progress in another process sharing the
cache survives. The README backup layout and TODO.md are updated.

Model: opus-5-5

TODO.md

@@ -18,6 +18,13 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Opening a library no longer deletes another process's download in
+  progress (issue 105). The download writer's temp files are named
+  `.quak-<pid>-<random>.tmp`, and `removeLeftoverTempFiles`, moved from the
+  backup into the download module, deletes a `.quak-*.tmp` file only when the
+  process ID in its name is no longer running. The content cache calls it at
+  `open()` for `originals/` and `thumbnails/`, the backup as before. Temp files
+  written before this change carry no process ID and are no longer deleted.
 - 2026-09-23: Fixed the backup's per-collection folders (issue 103). Two files
   in one collection with the same title, and two collections with the same name,
   each get their ID added to the name (`IMG_0001 (12345).JPG`, `Trip (10)/`), so