test(release): FakeBackend is a first-class BackendKind and desktop E2E material ships in release artifacts #3211

Found by a repo-wide simplification audit (static analysis; zero-consumer claims re-verified with git grep -aI).

FakeBackend is a test backend, but it is wired as first-class production surface, and desktop E2E material ships in release artifacts:

• BackendKind defines 'fake' as a formal session backend: packages/core/src/session.ts:262.
• The normal Runtime Host composition unconditionally registers it: packages/runtime-host/src/server/execution-composition.ts:289.
• Unknown/legacy providers fall back to 'fake': packages/core/src/llm-connections.ts:375.
• The production candidate entry statically imports the desktop E2E composition: packages/runtime-host/src/execution-candidate-main.ts (top-of-file imports).
• Release packaging copies whole dist/ trees, so FakeBackend, E2E fixture schema, and --desktop-e2e branches ship in the CLI package and the Electron app: scripts/release-cli-package.mjs:367-369, apps/desktop/electron-builder.config.mjs.

Product layers then carry fake special-cases (search exclusion in apps/desktop/src/main/search/thread-search.ts, model selector, stale-session handling). Note the fake-rejection in chat-readiness.ts (assertSessionCanSend) currently has no production caller — only errorMessage is imported by main-window.ts — so it is residue rather than an active guard.

Suggested direction: keep E2E going through the real composition via the existing primaryBackendFactory seam, but move FakeBackend + fixture schema + candidate bootstrap to a test-only compile entry, drop 'fake' from BackendKind and the unknown-provider fallback, and stop shipping fixture material in release artifacts.