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/settings-pairwise.test.ts

@@ -1,8 +1,7 @@
-import { readFileSync } from "node:fs";
-import { join } from "node:path";
 import { TOOLS } from "@snapotter/shared";
 import { afterAll, beforeAll, describe, expect, it } from "vitest";
 import { getToolConfig } from "../../apps/api/src/routes/tool-factory.js";
+import { fixtures, readFixture } from "../fixtures/index.js";
 import { pairwise } from "../helpers/pairwise.js";
 import { defaultSettingsFor } from "../helpers/tool-default-settings.js";
 import { compactCase, deriveAxes } from "../helpers/zod-pict.js";
@@ -41,7 +40,7 @@ describe("pairwise settings matrix", () => {
 
   beforeAll(async () => {
     testApp = await buildTestApp();
-    inputPng = readFileSync(join(__dirname, "..", "fixtures", "test-200x150.png"));
+    inputPng = readFixture(fixtures.image.base.png200);
   }, 30_000);
 
   afterAll(async () => {