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/src/boot.tsx

@@ -11,6 +11,7 @@ import { Context } from 'cordis'
 import { createRoot } from 'react-dom/client'
 import type { ReactNode } from 'react'
 import type { ObservableSnapshot } from '@deepseek-ai/dsh-client-web-react'
+import { createSlotRenderer } from '@deepseek-ai/dsh-client-web-react'
 import { createClientLoader, type ClientLoaderOptions } from '@deepseek-ai/dsh-client-runtime/loader'
 import { AppRoot } from './AppRoot.tsx'
 import { buildRenderApp } from './app.tsx'
@@ -59,7 +60,13 @@ export function bootWebShell(el: HTMLElement, seams?: BootSeams): () => void {
 
   loader.start()
   loader.settled().then(
-    () => { settled.flip() },
+    () => {
+      // The renderer install is a shell-boot act, but ctx.slots exists only
+      // once the runtime plugin loaded — so it lands here, after settled and
+      // before the flip that lets renderApp call renderSlot('root').
+      ctx.slots.install(createSlotRenderer())
+      settled.flip()
+    },
     () => { /* stay on the loading page; failures render from loader.status */ },
   )
   return () => { root.unmount() }