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/unit/api/format-encoders.test.ts

@@ -1,5 +1,4 @@
 import { readFile } from "node:fs/promises";
-import { join } from "node:path";
 import sharp from "sharp";
 import { describe, expect, it } from "vitest";
 import {
@@ -8,8 +7,7 @@ import {
   encodeJp2,
   encodeQoi,
 } from "../../../apps/api/src/lib/format-encoders.js";
-
-const FIXTURES = join(__dirname, "../../fixtures");
+import { fixtures, readFixture } from "../../fixtures/index.js";
 
 async function createTestPng(width = 50, height = 50): Promise<Buffer> {
   return sharp({
@@ -52,7 +50,7 @@ describe("encodeQoi", () => {
   });
 
   it("encodes fixture image to QOI", async () => {
-    const input = await readFile(join(FIXTURES, "test-200x150.png"));
+    const input = readFixture(fixtures.image.base.png200);
     const result = await encodeQoi(input);
     expect(result.length).toBeGreaterThan(100);
   });
@@ -170,7 +168,7 @@ describe("encodeJp2", () => {
   });
 
   it("produces valid output at different quality levels", async () => {
-    const input = await readFile(join(FIXTURES, "test-200x150.png"));
+    const input = readFixture(fixtures.image.base.png200);
     try {
       const low = await encodeJp2(input, 10);
       const high = await encodeJp2(input, 90);