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/ui-conversation/tests/chat-view.spec.tsx

@@ -3,7 +3,7 @@
 // toolview dispatch and selection handoff — driven through a scripted
 // ObservableSnapshot fake, no wire.
 
-import { afterEach, describe, expect, it, vi } from 'vitest'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { Profiler } from 'react'
 import { act, cleanup, fireEvent, render } from '@testing-library/react'
 import type {
@@ -13,10 +13,16 @@ import { bindSnapshotSelector } from '@deepseek-ai/dsh-client-web-react'
 import type { UseSession } from '@deepseek-ai/dsh-client-web-react'
 import type { ConvViewProps, SelectionTarget } from '@deepseek-ai/dsh-client-ui-conversation/client'
 import { ToolViewRegistry } from '@deepseek-ai/dsh-client-ui-conversation/client'
+import { createChatStore } from '../src/client/stores.ts'
 import { createChatView } from '../src/client/chat/ChatView.tsx'
 import { deriveChatFlow, flowKeys } from '../src/client/chat/chat-flow.ts'
 
 afterEach(cleanup)
+// Keyless create() persists under the bare declared key; clear between cases
+// so one harness's selection cannot rehydrate into the next.
+beforeEach(() => {
+  localStorage.clear()
+})
 
 const SID = 's1' as SessionId
 
@@ -68,33 +74,17 @@ function makeHarness(init?: Partial<ConversationSnapshot>) {
   const ChatView = createChatView({ toolviews: registry, t: (k) => k })
   const openDetails = vi.fn<(t: SelectionTarget) => void>()
   const loadOlder = vi.fn()
-  const selection = makeSelection()
+  // Selection rides the REAL chat store (same construction path as
+  // production; the view reads it through the ConvViewProps useStore share).
+  const chat = createChatStore().create()
   const props: ConvViewProps = {
     sessionId: SID,
     useSession: bindSnapshotSelector(source) as unknown as UseSession,
-    useSelection: bindSnapshotSelector(selection.source),
+    useStore: chat.useSelector,
     actions: { openDetails, loadOlder },
-    slots: { renderSlot: () => null } as never,
-  }
-  return { set, registry, ChatView, props, openDetails, loadOlder, setSelection: selection.set }
-}
-
-function makeSelection() {
-  let sel: SelectionTarget | null = null
-  const subs = new Set<() => void>()
-  return {
-    set(next: SelectionTarget | null) {
-      sel = next
-      for (const fn of [...subs]) fn()
-    },
-    source: {
-      getSnapshot: () => sel,
-      subscribe: (fn: () => void) => {
-        subs.add(fn)
-        return () => subs.delete(fn)
-      },
-    },
   }
+  const setSelection = (next: SelectionTarget | null): void => { chat.actions.select(next) }
+  return { set, registry, ChatView, props, openDetails, loadOlder, setSelection }
 }
 
 describe('chat-flow derivation', () => {