refactor(client): name the compile face in every client test filename

A test file under packages/client now says which face it covers:
`*.client.spec.{ts,tsx}` and its `*.client.{ts,tsx}` helpers belong to the
Client aggregate, `*.host.spec.ts` to the host aggregate. The carrier's four
node-half specs take the Host suffix.

The two suffixes are mutually exclusive, so each aggregate excludes the
other's and both keep one broad test glob: `exclude` wins over `include`, and
`packages/client/**` no longer has to be excluded wholesale from the host
program with per-file `files` entries carved back out of it. A Host-face spec
that reaches only Host source therefore needs no cross-face project
reference, which the split-project rule rejects.

vitest still discovers every file through `**/*.spec.{ts,tsx}`.

packages/client/runtime/tests/time-zone.client.spec.ts

@@ -0,0 +1,24 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { resolvedClientTimeZone } from '../src/client/time-zone.ts'
+
+afterEach(() => {
+  vi.restoreAllMocks()
+})
+
+describe('browser time zone', () => {
+  it('returns the runtime-resolved zone', () => {
+    expect(resolvedClientTimeZone()).toBe(
+      new Intl.DateTimeFormat().resolvedOptions().timeZone,
+    )
+  })
+
+  it.each([undefined, ''])('fails loud when the runtime exposes no zone %#', (timeZone) => {
+    const options = new Intl.DateTimeFormat().resolvedOptions()
+    vi.spyOn(Intl.DateTimeFormat.prototype, 'resolvedOptions').mockReturnValue({
+      ...options,
+      timeZone: timeZone as string,
+    })
+
+    expect(() => resolvedClientTimeZone()).toThrow('browser time zone is unavailable')
+  })
+})