refactor(data): split @adobe/data-testing into its own package

[dwabyick]

Summary

• @adobe/data's peerDependencies field applied to the whole package, not just the /testing subpath — so any project depending on @adobe/data picked up a vitest peer requirement even if it never touches conformance-testing helpers. Under strict peer-dependency resolution this could surface as a vitest version conflict for a consumer that has nothing to do with testing.
• The vitest dependency is real, not accidental: src/testing/conformance/*.ts calls describe/it from vitest at module scope, and those calls must resolve to the same vitest instance running the consumer's own test suite (module-identity requirement for test registration) — a plain dependency would risk a duplicate/mismatched vitest instance, and a devDependency wouldn't install at all for downstream consumers.
• Fix: moved src/testing/** (the Match and Conformance namespaces) out of @adobe/data into a new workspace package, @adobe/data-testing, which now carries the vitest peer dependency itself. @adobe/data no longer mentions vitest outside its own devDependencies.
• Updated the 7 sample packages that actually author conformance tests (data-lit-todo, data-lit-tictactoe, data-lit-space-rock-game, data-gpu-hopper, data-p2p-tictactoe, data-react-pixie, data-solid-dashboard) to import from @adobe/data-testing and depend on it directly.
• Updated the @adobe/data-ai AI-guidance rule docs (state.md, conformance.md, computed.md) that referenced the old @adobe/data/testing import path.

Note on the lockfile diff

pnpm-lock.yaml's format bump (lockfileVersion 6.0 → 9.0) is a side effect of running pnpm install with the CI-pinned pnpm major version (9) against a lockfile last written by pnpm 8 — any real (non-frozen) install with pnpm 9 upgrades the format on first write. It's unrelated to this change in substance but unavoidable once the lockfile needs updating for the new package.

Test plan

• pnpm -r run typecheck — all 18 packages pass, including the new @adobe/data-testing
• pnpm -r run build — all packages build clean
• pnpm -r run test — all packages pass (245 tests in @adobe/data, 10 in @adobe/data-testing, and the full suite in every consumer sample app)
• pnpm run check:workspace — workspace conventions OK
• pnpm -r run lint — clean

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