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-branch-tails.spec.tsx

@@ -9,8 +9,8 @@ import { cleanup, render } from '@testing-library/react'
 import { act } from '@testing-library/react'
 import type { SessionId, ToolResultNode } from '@deepseek-ai/dsh-client-runtime/client'
 import type { RpcId } from '@deepseek-ai/dsh-client-connection/client'
-import { bindSnapshotSelector, createSessionProvider } from '@deepseek-ai/dsh-client-web-react'
-import type { SessionBinding as ReactSessionBinding, UseSession } from '@deepseek-ai/dsh-client-web-react'
+import { bindSnapshotSelector } from '@deepseek-ai/dsh-client-web-react'
+import type { UseSession } from '@deepseek-ai/dsh-client-web-react'
 import { ToolViewRegistry } from '@deepseek-ai/dsh-client-ui-conversation/client'
 import type { ToolViewProps, Translate } from '@deepseek-ai/dsh-client-ui-conversation/client'
 import { MessageItem } from '../src/client/chat/MessageItem.tsx'
@@ -92,34 +92,33 @@ describe('small branch tails', () => {
 })
 
 describe('ToolViewOutlet dispatch', () => {
-  it('caches the inject factory per (registration x binding) and merges its props', () => {
+  it('caches the inject factory per (registration x session) and merges its props', () => {
     const registry = new ToolViewRegistry()
-    const inject = vi.fn(() => ({ extra: 'injected' }))
+    const inject = vi.fn((sessionId: SessionId) => ({ extra: `injected:${sessionId}` }))
     registry.register('bash',
       (p: ToolViewProps & { extra: string }) => <div data-testid="row">{p.extra}</div>,
       { inject })
-    // InjectedRow reads the session binding from context: mount through the
-    // real SessionProvider so the (factory x binding) cache path executes.
-    const binding: ReactSessionBinding = {
-      sessionId: SID,
-      session: { useSelector: (() => { throw new Error('unused') }) as never },
-      ctx: {},
-    }
-    const Provider = createSessionProvider({
-      useCurrent: () => SID,
-      resolveBinding: () => binding,
-      renderBody: () => (
-        <ToolViewOutlet registry={registry} sessionId={SID} toolName="bash" viewProps={viewProps()} />
-      ),
-    })
-    const view = render(<Provider />)
-    expect(view.getByTestId('row').textContent).toBe('injected')
+    // Pure props machinery: the outlet feeds its own sessionId to the
+    // factory — no provider/context needed (terminal channel form).
+    const view = render(
+      <ToolViewOutlet registry={registry} sessionId={SID} toolName="bash" viewProps={viewProps()} />,
+    )
+    expect(view.getByTestId('row').textContent).toBe(`injected:${SID}`)
     expect(inject).toHaveBeenCalledTimes(1)
-    // Remount against the SAME binding: cache hit, factory not re-run.
+    // Remount under the SAME session: cache hit, factory not re-run.
     view.unmount()
-    const second = render(<Provider />)
-    expect(second.getByTestId('row').textContent).toBe('injected')
+    const second = render(
+      <ToolViewOutlet registry={registry} sessionId={SID} toolName="bash" viewProps={viewProps()} />,
+    )
+    expect(second.getByTestId('row').textContent).toBe(`injected:${SID}`)
     expect(inject).toHaveBeenCalledTimes(1)
+    // A different session is a distinct cache key: factory runs once more.
+    second.unmount()
+    const other = render(
+      <ToolViewOutlet registry={registry} sessionId={'s2' as SessionId} toolName="bash" viewProps={viewProps()} />,
+    )
+    expect(other.getByTestId('row').textContent).toBe('injected:s2')
+    expect(inject).toHaveBeenCalledTimes(2)
   })
 
   it('a crashing custom row falls back to GenericToolCard and retries on re-registration', () => {