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/library/content.test.ts

@@ -33,6 +33,7 @@ import {
     mkdirSync,
     statSync,
 } from "node:fs";
+import { spawnSync } from "node:child_process";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 
@@ -161,15 +162,28 @@ describe("ContentCache.open", () => {
         expect(statSync(thumbnails).mode & 0o777).toBe(0o700);
     });
 
-    it("reaps orphan temp files but keeps complete content", async () => {
+    it("removes temp files of an exited process, keeping those of a running one and complete content", async () => {
         const originals = join(cacheDir, "originals");
         const thumbnails = join(cacheDir, "thumbnails");
         mkdirSync(originals, { recursive: true });
         mkdirSync(thumbnails, { recursive: true });
-        const orphan = join(originals, ".quak-abc123.tmp");
+        // A child that has already exited: its process ID is not running.
+        const exitedPID = spawnSync(process.execPath, ["-e", ""]).pid;
+        const orphan = join(originals, `.quak-${exitedPID}-abc123.tmp`);
+        const orphanThumb = join(thumbnails, `.quak-${exitedPID}-abc456.tmp`);
+        // This test's own process stands in for another process still
+        // downloading into the same cache.
+        const inProgress = join(originals, `.quak-${process.pid}-def123.tmp`);
+        const inProgressThumb = join(
+            thumbnails,
+            `.quak-${process.pid}-def456.tmp`,
+        );
         const complete = join(originals, "1.jpg");
         const thumb = join(thumbnails, "2.jpg");
         writeFileSync(orphan, "half-written");
+        writeFileSync(orphanThumb, "half-written");
+        writeFileSync(inProgress, "half-written");
+        writeFileSync(inProgressThumb, "half-written");
         writeFileSync(complete, "whole");
         writeFileSync(thumb, "whole-thumb");
 
@@ -177,8 +191,12 @@ describe("ContentCache.open", () => {
         await cache.open();
 
         expect(existsSync(orphan)).toBe(false);
+        expect(existsSync(orphanThumb)).toBe(false);
+        expect(existsSync(inProgress)).toBe(true);
+        expect(existsSync(inProgressThumb)).toBe(true);
         expect(existsSync(complete)).toBe(true);
         expect(existsSync(thumb)).toBe(true);
+        expect(cache.pathsFor(1)).toEqual({ originalPath: complete });
     });
 
     it("records already-cached files so their paths appear in pathsFor", async () => {