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.

tsconfig.base.json

@@ -118,6 +118,7 @@
       "@deepseek-ai/dsh-client-modules": ["./packages/client/modules/src"],
       "@deepseek-ai/dsh-client-runtime": ["./packages/client/runtime/src"],
       "@deepseek-ai/dsh-client-runtime/client": ["./packages/client/runtime/src/client"],
+      "@deepseek-ai/dsh-client-test-runtime": ["./packages/client/test-runtime/src"],
       "@deepseek-ai/dsh-client-ui-layout": ["./packages/client/ui-layout/src"],
       "@deepseek-ai/dsh-client-ui-sidebar": ["./packages/client/ui-sidebar/src"],
       "@deepseek-ai/dsh-client-ui-conversation": ["./packages/client/ui-conversation/src"],