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

test/download/download.test.ts

@@ -1563,7 +1563,13 @@ describe("writeAtomic", () => {
         // directory so that new entry is on disk too. Do the directory fsync
         // before the rename, or skip it, and a crash can lose the rename.
         expect(durabilityHook.events).toHaveLength(3);
-        expect(durabilityHook.events[0]).toMatch(/^sync:w:.*\.tmp$/);
+        // The temp name carries this process's ID, so a library opening the
+        // same cache can tell a write in progress from a leftover.
+        const tempSync = durabilityHook.events[0]!;
+        expect(tempSync.startsWith(`sync:w:${dir}/`)).toBe(true);
+        expect(tempSync.slice(`sync:w:${dir}/`.length)).toMatch(
+            new RegExp(`^\\.quak-${process.pid}-[0-9a-f]{32}\\.tmp$`),
+        );
         expect(durabilityHook.events[1]).toBe(`rename:${dest}`);
         expect(durabilityHook.events[2]).toBe(`sync:r:${dir}`);
     });