Allow contract tests a cold plugin cache

The contract tests run the debug binary, whose first run compiles the
bundled plugins into an empty wasmtime cache. CI used to warm that cache
by running the CLI tests first; without that step the first contract test
took 6 s on a CI runner and hit bun's 5 s default timeout.

AI assistance: written with Claude Code; reviewed by the author.

Claude-Session: https://claude.ai/code/session_01KDZ845xr2ngWfAabx9dg8z
Agent-Session: dca03de1-4350-4b24-b376-cb54c86fcc2d
Agent-Session: 82924cb5-dcfa-42e4-abb0-ef40393249f8
Agent-Session: 01a103d9-eefe-7d11-9db3-da97527a7db7

Author: sidkmenon

diffr-ts/src/contract.test.ts

@@ -1,4 +1,4 @@
-import { afterEach, describe, expect, test } from "bun:test";
+import { afterEach, describe, expect, setDefaultTimeout, test } from "bun:test";
 import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
@@ -10,6 +10,10 @@ import {
 } from "./index.js";
 import { diffrBinary, repositoryRoot } from "./test-binary.js";
 
+// These tests run the debug binary. Its first run compiles the bundled plugins
+// into an empty wasmtime cache, which is slow on CI runners.
+setDefaultTimeout(30_000);
+
 const dirs: string[] = [];
 function tempDir(prefix: string): string {
   const dir = mkdtempSync(join(tmpdir(), prefix));