feat(session,web): 会话角色不可变 — PATCH 拒 personaId + 移除前端换角色入口

design/22 §8-1（主人 2026-10-03 拍板）：会话角色在创建时确定、之后不可变，
要换角色必须新开会话。

后端：
- PATCH /api/v1/sessions/{id} 的 personaId 改为 json.RawMessage 判「键出现」，
  出现即 400（null/""/未知 id 一律拒），且先于会话存在性检查；错误消息点名 personaId
- 删除 Server.setSessionPersona 与 PATCH 的 501 分支；title/pinned 行为逐字不变
- sessionPersonaWriter 接口与 history.Store.SetSessionPersona 保留（存储能力）

前端：
- 移除会话行「切换角色」入口与面板「挂角色」下拉（含 setSessionPersona action、
  lib/sessionPersona.ts）；保留「新建会话选角色」；面板「角色」列改为只读展示

契约：Docs/api-client.md -> v0.46（破坏性：依赖 PATCH 换角色的客户端须改为新建会话）

验证：go build/vet ./... 绿；gateway+history 包测试绿（-count=1）；前端 npm run build
（tsc --noEmit + vite build）绿；9 个前端逻辑测试全绿。
变异验证：改回「静默忽略」/「写绑定」两种反向实现，新断言均 FAIL。

frontend/web/src/lib/api/sessions.ts

@@ -75,17 +75,17 @@ export function createSubSession(opts: {
   });
 }
 
-/** One PATCH /api/v1/sessions/{id} change set. At least one field must be set;
- *  the gateway rejects an empty body with 400. `personaId: ""` explicitly
- *  re-binds the session to "inherit the default role" (design/22 §4.5). */
+/** One PATCH /api/v1/sessions/{id} change set. At least one field must be set; the
+ *  gateway rejects an empty body with 400. `personaId` is deliberately absent: a
+ *  session's role is fixed at creation (design/22 §8-1), so the gateway answers 400
+ *  for any body carrying that key — switching roles means creating a new session. */
 export interface SessionPatch {
   title?: string;
   pinned?: boolean;
-  personaId?: string;
 }
 
-/** PATCH /api/v1/sessions/{id} — updates a session's display metadata and/or its
- *  bound role. The reply echoes only the fields that were changed. */
+/** PATCH /api/v1/sessions/{id} — updates a session's display metadata (rename /
+ *  pin). The reply echoes only the fields that were changed. */
 export function patchSession(
   sessionId: string,
   patch: SessionPatch