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/locale/README.md

@@ -2,7 +2,7 @@
 
 English | [中文](README.zh.md)
 
-Locale plugin: LocaleService — the browser locale preference (`zh`/`en`, persisted under `dsh.locale`, getter/setter with `locale/change` snapshots) plus the ns×locale dictionary registry (`bind(ns)`→t with a stable function identity; lookup chain active → zh → key).
+Locale plugin: LocaleService — the browser locale preference (`zh`/`en`, persisted under `dsh.locale`; `locale/change` fires on switches only) plus the ns×locale dictionary registry (typed `register(ns, {zh, en})` checked against `LocaleNamespaceMap`, `bind(ns)`→`TranslateNS<ns>`; lookup chain ns → common → zh → key). The service implements the slot system's `LocaleFace` and installs itself through `ctx.slots.installLocale`, backing the framework-injected `t` standard seat (`Translate`/`TranslateNS` are ui-slots types; import them from there — this package only re-exports for dictionary owners' convenience).
 
 ## Model Experience
 
@@ -14,5 +14,5 @@ None; this package neither assembles nor sends a provider request.
 
 ## Known Limitations and Deferred Work
 
-- **Only the Settings surface is translated** — other pages keep inline copy; repo-wide extraction into dictionaries is deferred.
-- **Locale switching re-renders subscribed consumers only** — sections not wired to `locale/change` keep their rendered text until remount.
+- **Most surfaces keep inline copy** — the standard seat is adopted by the Settings rows, sidebar, question composer, and model select; the remaining packages migrate in follow-up PRs.
+- **Registry-held text reads its translation once** — copy captured at registration time outside the slot render path (e.g. the `/model` command description in the command registry) keeps the language it was registered under until re-registration; slot-rendered copy follows switches live.