Publish the adapter contract suite as a reusable test kit

[camcima]

Deferred from the architecture review (finding L7) — see ARCHITECTURE_REVIEW.md.

Problem

packages/core/tests/contract/adapter-contract.test.ts is what keeps the four built-in adapters behaviorally interchangeable. Every adapter package imports it by relative path:

import { runAdapterContractTests } from "../../../core/tests/contract/adapter-contract.test.js";

That works inside the monorepo and nowhere else. @ziggurat-cache/core ships only dist and README.md, so the suite is not on npm.

Meanwhile docs/custom-adapters.md invites people to build their own CacheAdapter. They cannot run the compliance suite that the first-party adapters are held to — the doc currently just lists the behaviors in prose and asks them to reproduce it by hand.

For a library whose selling point is a modular adapter ecosystem, this is the gap worth closing: it turns "implement this interface" into "pass this suite."

Sketch

Publish the suite as @ziggurat-cache/adapter-testkit:

• New package exporting runAdapterContractTests(name, factory, options).
• vitest moves to a peer dependency — consumers bring their own runner.
• The four adapter packages import the published entry point instead of reaching across directories, which also removes the rootDir TypeScript complaints those relative imports currently produce.
• docs/custom-adapters.md replaces its prose checklist with real usage, and drops the "not published yet" caveat added in docs: separate the dev and consumer Node floors #57.

Done when

• An adapter developed outside this repo can npm i -D @ziggurat-cache/adapter-testkit and run the same suite CI runs here.
• No adapter package imports across package boundaries by relative path.
• Release wiring covers the new package.

Notes

Deliberately left out of #56 because it is a new published package — build config, release wiring, docs — rather than a fix to existing code.