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.

scripts/verify-package-readme-model-experience.ts

@@ -47,6 +47,7 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/code-runtime/code-runtime-worker': { kind: 'indirect', reason: 'The worker backend delegates model rendering to Code Mode in dsh-tools.' },
   'packages/client/hmr': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/client/modules': { kind: 'none', reason: 'Browser-side module-loading kernel machinery; registers no model surface.' },
+  'packages/client/test-runtime': { kind: 'none', reason: 'Browser-side test infrastructure (jsdom bench); registers no model surface.' },
   'packages/client/ui-slots': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/client/ui-primitives': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },
   'packages/client/web-react': { kind: 'none', reason: 'Browser-side UI plugin layer; registers no model surface.' },