test: reorganize fixture files into modality-first layout (phase 6b)

Move all fixture files from flat/mixed dirs (content/, media/, documents/,
formats/, hostile/, root loose) into the modality-first hierarchy:
image/{valid,formats,edge,hostile}, video/{valid,formats,hostile},
audio/{valid,formats,hostile}, document/{valid,formats,edge,hostile},
data/valid/, security/. Update index.ts paths, fixtureDir aliases,
all literal refs in 17 e2e/qa/script files, manifest.json, and the
three generator scripts. 163 files moved, 0 dropped, 100 new tests
from expanded document scan.

tests/fixtures/image/edge/test-image-urls.txt

@@ -0,0 +1,30 @@
+https://picsum.photos/id/10/800/600
+https://picsum.photos/id/20/1024/768
+https://picsum.photos/id/37/800/600
+https://picsum.photos/id/42/640/480
+https://picsum.photos/id/55/1280/720
+https://picsum.photos/id/100/800/800
+https://picsum.photos/id/119/500/500
+https://picsum.photos/id/137/1920/1080
+https://picsum.photos/id/160/600/400
+https://picsum.photos/id/180/300/300
+https://picsum.photos/id/200/800/600
+https://picsum.photos/id/237/400/300
+https://picsum.photos/id/256/1024/768
+https://picsum.photos/id/292/800/600
+https://picsum.photos/id/338/640/480
+https://picsum.photos/id/366/1280/720
+https://picsum.photos/id/399/800/800
+https://picsum.photos/id/433/500/500
+https://picsum.photos/id/501/1920/1080
+https://picsum.photos/id/550/600/400
+https://picsum.photos/seed/snapotter1/800/600
+https://picsum.photos/seed/snapotter2/1024/768
+https://picsum.photos/seed/snapotter3/640/480
+https://picsum.photos/id/670/800/600?grayscale
+https://picsum.photos/id/700/1024/768?blur=2
+https://picsum.photos/id/870/800/600?grayscale&blur=2
+https://picsum.photos/id/1002/400/400
+https://picsum.photos/id/1015/800/600
+https://picsum.photos/id/1025/1280/720
+https://picsum.photos/id/1043/1920/1080