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/integration/tools/audio/transcribe-audio.test.ts

@@ -9,7 +9,7 @@
 
 import { readFileSync } from "node:fs";
 import { afterAll, beforeAll, describe, expect, it } from "vitest";
-import { fixtureDir, fixtures } from "../../../fixtures/index.js";
+import { fixtures } from "../../../fixtures/index.js";
 import {
   buildTestApp,
   createMultipartPayload,
@@ -17,7 +17,6 @@ import {
   type TestApp,
 } from "../../test-server.js";
 
-const MEDIA = fixtureDir.media;
 const MP3 = readFileSync(fixtures.audio.tiny("mp3"));
 
 let testApp: TestApp;