A default name cut short is never a Windows device name

defaultFileName replaced unsafe characters before cutting the name short,
so the cut could leave a device name: `CONSOLE.` before a 251-letter
extension became `CON.` and that extension, and `NULL.` became `NUL.`. A
name the cut leaves as a device name now gets a leading `_`, as
sanitizeFileName gives one, and is cut a byte shorter to leave room for it.

The backup's links and album folders and the `backup-metadata` album
folders need no change: the ID comes right after or right before the part
that is cut, so that part can never be a device name.

Model: opus-5-5

src/filename.ts

@@ -92,7 +92,10 @@ export const shortenKeepingExtension = (
 // short so that the whole fits in a file name, keeping the title's extension
 // whenever it fits. `ext` (".mov"), when given, takes the place of the
 // title's extension before the name is cut short; a live photo's image and
-// video are each named with their own.
+// video are each named with their own. The cut can leave a device name
+// (`CONSOLE.<long extension>` becomes `CON.<long extension>`); that name gets
+// a leading `_`, as `sanitizeFileName` gives one, and is cut a byte shorter to
+// leave room for it.
 export const defaultFileName = (
     title: string,
     fileID: number,
@@ -101,11 +104,8 @@ export const defaultFileName = (
     const { prefix = "", ext } = opts;
     const safe = sanitizeFileName(title, `file-${fileID}`);
     const name = ext === undefined ? safe : withExtension(safe, ext);
-    return (
+    const room = MAX_NAME_BYTES - Buffer.byteLength(prefix);
-        prefix +
+    const cut = prefix + shortenKeepingExtension(name, room);
-        shortenKeepingExtension(
+    if (!RESERVED_DEVICE_NAME.test(cut)) return cut;
-            name,
+    return `_${prefix}${shortenKeepingExtension(name, room - 1)}`;
-            MAX_NAME_BYTES - Buffer.byteLength(prefix),
-        )
-    );
 };

test/filename/filename.test.ts

@@ -216,4 +216,13 @@ describe("defaultFileName", () => {
             `${"t".repeat(250)}.heic`,
         );
     });
+
+    // Cutting `CONSOLE` or `NULL` short before a long extension leaves `CON`
+    // or `NUL`, so the name gets the `_` `sanitizeFileName` gives a device
+    // name, and the part before the extension gives up a byte for it.
+    it("never returns a Windows device name, even once cut short", () => {
+        const ext = `.${"e".repeat(251)}`;
+        expect(defaultFileName(`CONSOLE${ext}`, 42)).toBe(`_CO${ext}`);
+        expect(defaultFileName(`NULL${ext}`, 42)).toBe(`_NU${ext}`);
+    });
 });