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/tests/__snapshots__/runtime.spec.tsx.snap

@@ -0,0 +1,37 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`single-slot mounting (declare + renderSlot) > folds class hashes and collapses svg internals in snapshots, leaving the live DOM alone 1`] = `
+<div
+  data-slot="trt.panel"
+>
+  <div
+    class="frame plain"
+  >
+    <span
+      class="label"
+    >
+      styled
+    </span>
+    <svg
+      aria-hidden="true"
+      data-content="2bfa09dc"
+      viewBox="0 0 16 16"
+    />
+  </div>
+</div>
+`;
+
+exports[`single-slot mounting edge arms > serializes childless svg untouched next to scoped classes 1`] = `
+<div
+  data-slot="trt.panel"
+>
+  <div
+    class="frame"
+  >
+    <svg
+      aria-hidden="true"
+      viewBox="0 0 1 1"
+    />
+  </div>
+</div>
+`;