Harden the backup tree's atomic copy (closes #22)

The backup copy now fsyncs its temp file before the rename and the
directory after it, using the download writer's new fsyncPath helper.
Each backup run deletes .quak-backup-*.tmp files whose process is no
longer running, leaving those of a concurrent backup alone. The rename
sites and the README backup layout state that a symlink at the
destination is replaced and the new file takes the temp file's
permissions, and the README names the temp files. Adds tests for a
missing and an unwritable destination directory for downloadFile and
downloadThumbnail.

Model: opus-5-5

test/download/download.test.ts

@@ -48,6 +48,7 @@
  */
 
 import {
+    chmodSync,
     existsSync,
     mkdirSync,
     readdirSync,
@@ -989,6 +990,51 @@ describe.each(entryPoints)(
             expect(readFileSync(outPath)).toEqual(Buffer.from(existing));
             expect(readdirSync(dir)).toEqual(["rename-fails.bin"]);
         });
+
+        it("fails without creating anything when the destination directory does not exist", async () => {
+            const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+            const { header, ciphertext } = encryptFileBody(
+                patternBytes(64, 33),
+                key,
+            );
+            const { api, file } = fixtureFor(key, header, ciphertext);
+            const dir = freshDir();
+            const outPath = join(dir, "missing", "never.bin");
+
+            await expect(download(api, file, outPath)).rejects.toMatchObject({
+                code: "ENOENT",
+            });
+
+            // The missing directory is not created on the caller's behalf.
+            expect(readdirSync(dir)).toEqual([]);
+        });
+
+        // Root ignores directory permissions, so this cannot fail as root
+        // (the Docker test image runs as root).
+        it.skipIf(process.getuid?.() === 0)(
+            "fails without creating anything when the destination directory is not writable",
+            async () => {
+                const key =
+                    sodium.crypto_secretstream_xchacha20poly1305_keygen();
+                const { header, ciphertext } = encryptFileBody(
+                    patternBytes(64, 34),
+                    key,
+                );
+                const { api, file } = fixtureFor(key, header, ciphertext);
+                const dir = freshDir();
+                const outPath = join(dir, "never.bin");
+                chmodSync(dir, 0o500);
+                try {
+                    await expect(
+                        download(api, file, outPath),
+                    ).rejects.toMatchObject({ code: "EACCES" });
+                } finally {
+                    chmodSync(dir, 0o700);
+                }
+
+                expect(readdirSync(dir)).toEqual([]);
+            },
+        );
     },
 );