feat(client): add the dsh-client-test-runtime package

A jsdom slot test runtime for feature specs: a real Cordis Context, the
production SlotsService and web-react renderer, and typed session/workspace
doubles (TestSessions implements ISessions with FixtureSession sessions;
TestWorkspaces implements IWorkspaces), so the compiler flags fixture drift
when a production face changes. Fixtures feed plain data: list rows,
conversation snapshots, and ISession-typed behavior stubs; provide-bundle
materialization runs the shared SessionProvideChannel.

DOM snapshot support: declare()/renderSlot() mount a single slot inside a
data-slot wrapper for local .snap capture, and a snapshot serializer folds
CSS-module class hashes to their semantic locals and collapses svg internals
to a content fingerprint. The typed provide() constrains declared-service
fakes to Partial of the service's outward face.

packages/client/test-runtime/src/invariant.ts

@@ -0,0 +1,32 @@
+/**
+ * Package-owned invariant companion for `@deepseek-ai/dsh-client-test-runtime`.
+ * @module @deepseek-ai/dsh-client-test-runtime/invariant
+ */
+
+/* jscpd:ignore-start */
+import type { Context } from 'cordis'
+import type { InvariantInstaller } from '@deepseek-ai/dsh-invariants'
+
+const PACKAGE_NAME = '@deepseek-ai/dsh-client-test-runtime'
+
+/** Cordis companion plugin name. */
+export const name = 'client-test-runtime-invariant'
+/** Service required before the companion can reserve package ownership. */
+export const inject = ['invariants']
+
+/**
+ * No runtime invariant: this test-support package owns no production event
+ * stream or mutable data — it assembles the runtime SlotsService and renderer
+ * (whose packages own their invariants) around test doubles; its own behavior
+ * is exercised by its package tests.
+ */
+const install: InvariantInstaller = () => {}
+
+/**
+ * Register this package's invariant companion.
+ * @param ctx - Cordis context carrying the invariant service.
+ * @returns the installed registration's disposer after setup succeeds.
+ */
+export const apply = (ctx: Context): Promise<() => void> =>
+  Promise.resolve(ctx.invariants.register(PACKAGE_NAME, install))
+/* jscpd:ignore-end */