refactor(gui): slot system standard — single register, four props shares, framework store seat

The definitive slot model for the web client, replacing the first-generation
define/register two-step, ScopedSlots whitelist faces, and binding handles:

- 'root' is the only a-priori slot (SlotsService built-in); the shell renders
  exactly ctx.slots.renderSlot('root', {}).
- register is the single API: children = slot declaration + render
  authorization + runtime spec in one options object; misconfiguration fails
  loud at load (duplicate declaration, undeclared contribution, one store
  handle under two scopes).
- Component props arrive in four auto-derived shares: PropsRuntime<K>
  (owner params + session/global standard kits via declare-merge),
  PropsRenderSlots<S>, PropsStore<H>, and the inject business face.
  sessionId is framework-supplied; hooks are framework-made only.
- Framework store seat: defineStore factories declare schema/actions/persist;
  read = useStore, write = baked actions only; store scope derives from the
  mounting entry; per-session persist keys and clearPersisted lifecycle.
- inject factories read the apply closure's own ctx (binding handles retired;
  root-ctx back door closed); SessionProvider is self-wired render-prop.
- Rendering sits behind the SlotRenderer install seam; runtime stays
  React-free; ownership ledger keyed to the single entry axis closes the
  stale-authority window (StaleAuthorizationError probes).

Docs: the slot type-chain note is refreshed in place as the slot system
standard RFC (bilingual pair re-recorded); the web client architecture RFC
defers its slot sections there; packages/client/AGENTS.md gains the slot and
props discipline; gui-testing/web-styling notes drop missions/ references.

Tests: suites rewritten to the standard (props fed directly, real store
engines via createXXXStore().create(), no render machinery); load-time
negative samples for declaration/authorization/store conflicts; verified by
real-host playwright run (three columns, empty state, collapse, keyed session
remount, cross-slot selection sharing).

docs(ui-sidebar): point contract reference at the committed slot standard RFC

missions/ is workspace-local and never committed; the README must not cite it.

packages/client/web-react/README.md

@@ -1,10 +1,10 @@
 # @deepseek-ai/dsh-client-web-react
 
-ctx↔React glue: createSnapshotStore (zustand vanilla + immer + subscribeWithSelector + rafFlush + opt-in persist), bindSnapshotSelector, SessionProvider (dependency-inverted), scopedSlots outlet, RootBindingProvider, useInvoke. Contract: api-contracts v3 §2.
+ctx↔React machinery for the slot terminal design: createSlotRenderer (the SlotRenderer implementation the shell installs into the runtime SlotsService), SessionProvider (framework-wired render prop over the host's current-session source), defineStore (the declarative store shell over the internal zustand engine), bindSnapshotSelector, useInvoke. The snapshot-store engine (createSnapshotStore) is framework-internal via the `./store` subpath; business plugins declare stores through defineStore only.
 
 ## Model Experience
 
-None, as the ctx↔React glue runs entirely in the browser; nothing here reaches a model request.
+None, as the ctx↔React machinery runs entirely in the browser; nothing here reaches a model request.
 
 #### KV Cache effect
 
@@ -12,6 +12,6 @@ None; this package neither assembles nor sends a provider request.
 
 ## Known Limitations and Deferred Work
 
-- **The persist middleware corrupts primitive-state stores** — it object-spreads state on save, so a `SnapshotStore<string>` round-trips as a character map; consumers with primitive state hand-roll persistence instead (ui-conversation drafts is the precedent).
+- **The persist middleware corrupts primitive-state stores** — it object-spreads state on save, so a `SnapshotStore<string>` round-trips as a character map; the engine hand-rolls persistence instead (see `attachPersistence`).
 - **`UseSession` is deliberately wide (`object` snapshot)** — the dependency direction (runtime → web-react, never the reverse) keeps the real `ConversationSnapshot` type out of reach; session-slot consumers narrow once at their boundary.
 - **renderSlot is the single P-I form** — no Suspense, no per-entry lazy loading; the progressive-rendering surface returns with its own project.