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/bg-effects.test.ts

@@ -1,4 +1,3 @@
-import { join } from "node:path";
 import sharp from "sharp";
 import { describe, expect, it } from "vitest";
 import {
@@ -10,8 +9,6 @@ import {
   createGradientBackground,
 } from "../../../apps/api/src/lib/bg-effects.js";
 
-const _FIXTURES = join(__dirname, "../../fixtures");
-
 async function createTestImage(
   width: number,
   height: number,