test(ui-agent-preset): cover the surface where it is introduced

The package landed here with one store spec; the rest of its tests were written
three layers up, so this layer and the two above it failed the per-file
coverage gate on 191 locations while every test passed.

Adds the node half's invariant companion, the two components, the registration
apply, and the stores' failure paths. `currentValue` drops a `?? ''` that an
empty roster already returned before reaching — the same shape the later layer
arrived at, so the two converge instead of conflicting.

packages/client/ui-agent-preset/tests/invariant.spec.ts

@@ -0,0 +1,25 @@
+/** The package's node half: an empty host body and an explained empty invariant companion. */
+
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import InvariantService from '@deepseek-ai/dsh-invariants'
+import * as AgentPresetInvariant from '@deepseek-ai/dsh-client-ui-agent-preset/invariant'
+
+describe('invariant companion', () => {
+  it('reserves package ownership with an empty installer', async () => {
+    const ctx = new Context()
+    await ctx.plugin(InvariantService, { enabled: true })
+
+    await expect(ctx.plugin(AgentPresetInvariant).await()).resolves.toBeDefined()
+  })
+
+  it('has an empty node half', async () => {
+    const { apply } = await import('@deepseek-ai/dsh-client-ui-agent-preset')
+
+    // The host body exists only so the plugin appears in the host cordis.yml;
+    // every surface this package ships lives in the browser half.
+    apply()
+
+    expect(typeof apply).toBe('function')
+  })
+})