feat(client): typed locale standard seat in the slot framework

Registrations declare a dictionary namespace (locale: NS) and the renderer
synthesizes a typed t prop for the entry's component from the installed
LocaleFace; the seat binding is re-derived per locale revision, so a language
switch hands out fresh t references and memoized consumers re-render through
ordinary shallow comparison. LocaleNamespaceMap is the declare-merge table
(namespace -> dictionary key union); TranslateNS<'ns'> is the
namespace-addressed translate type (namespace keys plus the shared common
vocabulary), carried by the t seat and by the locale service's typed bind.

LocaleService implements the face (lookup ns -> common -> zh -> key,
revision-carrying snapshots with subscriber isolation) and installs it
through the boot-once slots.installLocale seam, mirroring the renderer
install. The typed register(ns, {zh, en}) overload checks each dictionary
against the namespace's key union and requires every shipped locale, so a
missing or extra key and an unbalanced translation are compile errors.
Dictionary registration bumps the face revision without emitting
locale/change — the event now means exactly 'the active locale switched',
so registration-heavy boot cannot storm event listeners.

packages/client/ui-sidebar/tests/sidebar-snapshot.spec.tsx

@@ -11,6 +11,7 @@
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import { cleanup, waitFor } from '@testing-library/react'
 import { SlotTestRuntime } from '@deepseek-ai/dsh-client-test-runtime'
+import { LocaleService } from '@deepseek-ai/dsh-client-locale/client'
 import { apply, inject } from '@deepseek-ai/dsh-client-ui-sidebar/client'
 
 afterEach(cleanup)
@@ -18,6 +19,12 @@ afterEach(cleanup)
 async function bench() {
   const runtime = await SlotTestRuntime.create()
   runtime.provide('layout', { toggleSidebar: vi.fn() })
+  // English locale pins the snapshots to the copy they were recorded with;
+  // the installed face backs the entry's standard `t` seat.
+  const locale = new LocaleService(runtime.ctx)
+  locale.setLocale('en')
+  runtime.provide('locale', locale)
+  runtime.slots.installLocale(locale)
   await runtime.declare({ 'sidebar': { kind: 'single', scope: 'root' } })
   await runtime.mount({ inject: [...inject], apply })
   return runtime