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/qa/editor.qa.spec.ts

@@ -496,7 +496,7 @@ test.describe("Editor Filters & Adjustments", () => {
     // Load a test image via route interception
     await page.route("**/test-fixture.png", (route) =>
       route.fulfill({
-        path: "tests/fixtures/test-200x150.png",
+        path: "tests/fixtures/image/valid/test-200x150.png",
         contentType: "image/png",
       }),
     );