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-layout/README.md

@@ -2,7 +2,7 @@
 
 Shell plugin: three-column AppFrame (drag handles, concession chain) + ctx.layout viewing-state service (nav, panel widths, persist); defines the sidebar/conversation/details/conversation.empty slots. Contract: api-contracts v3 §5.
 
-Slot declarations use the composed-props entry form (`owner` share, no full `props`): the exported OwnerShare contracts are `SidebarOwnerProps` / `ConvOwnerProps` / `DetailsOwnerProps` / `EmptyOwnerProps` — registrants reference them via `OwnerOf<'sidebar' | ...>` and compose their own injected share locally. No entry declares `children` (declaring it requires the registered component to carry the slots face): no P-I slot component delegates — `conversation.empty` is rendered by the shell's assembly closure, not handed down by ConversationRoot.
+Slot declarations use the composed-props entry form (`owner` share, no full `props`): the exported OwnerShare contracts are `SidebarOwnerProps` / `ConvOwnerProps` / `DetailsOwnerProps` / `EmptyOwnerProps` — registrants reference them via `OwnerOf<'sidebar' | ...>` and compose their own injected share locally. No entry declares `children` (declaring it requires the registered component to carry the slots face — reserved for future business slots): delegation authority is the component-side whitelist, i.e. AppFrame's `ScopedSlots<FrameSlotKey>` face over sidebar/conversation/details/conversation.empty. Since the root-slot rework the frame itself registers into 'root' and renders those child slots at its own render sites; the shell only renders 'root'.
 
 The export surface is the cross-package contract only: the AppFrame trio (+ `AppFrameProps`) consumed by the web shell's assembly, `LayoutService` with its store shapes (`NavState`/`PanelState`/`ViewId`), and the OwnerShare contracts. The concession-chain solver (`computeColumns`) and its geometry constants are package-internal; tests import them from `/src`.