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.

docs/web-styling.md

@@ -1,6 +1,6 @@
 # Web GUI 样式规范
 
-> **【token 体系已换代——§1 表格仅历史参考】** 本文的 `--bg-*`/`--text-*`/`--accent` token 族与其宿主包 `packages/client/web-ui` 已随插件化重构退役。现行 token 唯一来源=`packages/client/ui-theme/src/styles/` 的 `--dsw-*` 体系（static 色阶+alias 语义层，暗色=`body[data-ds-dark-theme]` 覆写）；组件对账基准=`missions/tasks/20260721-1520-web-plugin-rfc/style-spec.md`。**仍然有效**：工程约束（CSS Modules + clsx、无组件库、无 tailwind、组件禁 hardcode 色值）、字号成对写行高、间距 4 倍数、代码字体栈末位不放 monospace——这些已收编进 architecture.md §15。
+> **【token 体系已换代——§1 表格仅历史参考】** 本文的 `--bg-*`/`--text-*`/`--accent` token 族与其宿主包 `packages/client/web-ui` 已随插件化重构退役。现行 token 唯一来源=`packages/client/ui-theme/src/styles/` 的 `--dsw-*` 体系（static 色阶+alias 语义层，暗色=`body[data-ds-dark-theme]` 覆写），sheet 即权威、组件对账以它为准。**仍然有效**：工程约束（CSS Modules + clsx、无组件库、无 tailwind、组件禁 hardcode 色值）、字号成对写行高、间距 4 倍数、代码字体栈末位不放 monospace。
 
 > 状态：原「活文档」（随 `packages/client/web-ui` 演进）。视觉基线源自对 deepseekchat 前端仓的实测调研。框架决策与工程约束由 [web-styling-system RFC](../.agents/notes/implemented/process/2026-07-19-web-styling-system.md) 拍板，本文不重复论证。