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/split-pdf.test.ts

@@ -1,11 +1,11 @@
-import { readFileSync } from "node:fs";
 import { join } from "node:path";
 import { qpdfAvailable, qpdfPageCount } from "@snapotter/doc-engine";
 import AdmZip from "adm-zip";
 import { afterAll, beforeAll, describe, expect, it } from "vitest";
+import { fixtures, readFixture } from "../fixtures/index.js";
 import { buildTestApp, createMultipartPayload, loginAsAdmin, type TestApp } from "./test-server.js";
 
-const PDF = readFileSync(join(__dirname, "..", "fixtures", "test-3page.pdf"));
+const PDF = readFixture(fixtures.document.pdf3);
 
 let testApp: TestApp;
 let adminToken: string;