Pin three untested guards: download timer, URL fragment, short APP1 (closes #89)

The download idle deadline's timer is unref'd so it never holds the
process open, and a test checks no timer is left after a download
completes or fails. A test covers the rejection of "#" in a request
path. The EXIF scan accepts an APP1 segment only when its length is at
least 8, since a shorter one cannot hold the six-byte Exif header;
tests cover lengths 7 and 8.

Model: opus-5-5

src/metadata-backup.ts

@@ -45,8 +45,9 @@ export const extractExifFromJpeg = (
                 error: `segment length ${len} at byte ${offset} runs past the end of the file`,
             };
         if (marker === 0xe1) {
-            // APP1 — check for "Exif\0\0" header. A length under 8 has no
-            // room for it, and the bytes compared would be the next segment's.
+            // APP1 — check for "Exif\0\0" header. A length under 8 cannot hold
+            // the six-byte header, so the segment is not EXIF; below 6 the
+            // bytes compared would also lie past the segment.
             if (
                 len >= 8 &&
                 buf[offset + 4] === 0x45 &&

test/cli/metadata-exif.test.ts

@@ -52,12 +52,19 @@ describe("extractExifFromJpeg", () => {
     });
 
     it("ignores an APP1 segment too short to hold the Exif header", () => {
-        // Length 7 leaves room for "Exif\0" only. Without the length check
-        // the scan compared the header against bytes past the segment.
+        // A length under 8 cannot hold the six-byte "Exif\0\0" header, so the
+        // segment is not EXIF. This one has length 7 and holds only "Exif\0",
+        // which the old code, lacking the length check, returned as EXIF.
         const short = app1(EXIF_HEADER.slice(0, 5));
         expect(extractExifFromJpeg(bytes(SOI, short, SOS))).toEqual({});
     });
 
+    it("accepts an APP1 segment of length 8 holding just the Exif header", () => {
+        const scan = extractExifFromJpeg(bytes(SOI, app1(EXIF_HEADER), SOS));
+        expect(scan.error).toBeUndefined();
+        expect([...scan.exif!]).toEqual(EXIF_HEADER);
+    });
+
     it("reports a JPEG truncated inside a segment header", () => {
         const scan = extractExifFromJpeg(bytes(SOI, [0xff, 0xe1, 0x00]));
         expect(scan.exif).toBeUndefined();