test: migrate 207 test files to typed fixture registry (phase 5)

Codemod rewrites all five ad-hoc fixture-path styles to import from the
typed registry (tests/fixtures/index.ts):
- join(__dirname, "..", "fixtures", ...) inline patterns (100 files)
- const FIXTURES = join(__dirname, ...) + join(FIXTURES, ...) (92 files)
- process.cwd()-based paths (10 files)
- path.resolve(__dirname, ...) patterns (5 files)

Registry extended with 8 new keys (portraitJpg, portraitHeic,
motorcycle, svgLogo, barcodeAvif, qrAvif, crossFormatChat,
multipageTiff) and directory accessors (fixtureDir, fixtureRoot).

Dynamic directory scanners (format-matrix-generated,
format-matrix-multimodal, hostile-inputs) preserved via fixtureDir
imports. No fixtures swapped, bytes identical.

Parity gate: dropped 0, net-new 388.

tests/integration/preview-modality.test.ts

@@ -1,13 +1,13 @@
 import { readFile } from "node:fs/promises";
-import { join } from "node:path";
 import { resolveGs } from "@snapotter/doc-engine";
 import { ffmpegAvailable } from "@snapotter/media-engine";
 import { describe, expect, it } from "vitest";
 import { pdfFirstPagePreview, videoPosterPreview } from "../../apps/api/src/modality/preview.js";
+import { fixtures, readFixture } from "../fixtures/index.js";
 
 describe.skipIf(!ffmpegAvailable())("video poster preview (requires ffmpeg)", () => {
   it("renders a webp poster", async () => {
-    const buf = await readFile(join(process.cwd(), "tests/fixtures/media/tiny.mp4"));
+    const buf = readFixture(fixtures.video.tiny("mp4"));
     const poster = await videoPosterPreview(buf);
     expect(poster).not.toBeNull();
     expect(poster?.length).toBeGreaterThan(50);
@@ -16,7 +16,7 @@ describe.skipIf(!ffmpegAvailable())("video poster preview (requires ffmpeg)", ()
 
 describe.skipIf(!resolveGs())("pdf first-page preview (requires ghostscript)", () => {
   it("renders a png of page 1", async () => {
-    const buf = await readFile(join(process.cwd(), "tests/fixtures/test-3page.pdf"));
+    const buf = readFixture(fixtures.document.pdf3);
     const png = await pdfFirstPagePreview(buf);
     expect(png).not.toBeNull();
     expect(png?.subarray(1, 4).toString()).toBe("PNG");