Rebuild: an image without words around it is keyed as a stem

So a cover 'x.jpg' joins 'x.part01.rar' in a stem-named post; an image
set like 'IMG_008' keys as the stem 'IMG'. Both implementations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/release/identity.spec.ts

@@ -229,5 +229,7 @@ test.group('release / par2 names', () => {
     const par2 = id('(Geisterjäger John Sinclair Hörspiele Vol. 2) [59/59] - "Hörspiele Vol. 2.par2" yEnc', 'Hörspiele Vol. 2.par2')
     assert.equal(t1.key, t2.key)
     assert.equal(t1.key, par2.key)
+    // A cover without words around it joins its stem's archive.
+    assert.equal(id('[2/6] - "x.jpg" yEnc', 'x.jpg').key, id('[1/6] - "x.part01.rar" yEnc', 'x.part01.rar').key)
   })
 })