fix(web): fence plan mode projection races

packages/host/apiproxy/src/api/sessions.schema.ts

@@ -113,7 +113,10 @@ export const sessionCancelValueSchema = z.object({
 export const planModeStateSchema = z.object({
   active: z.boolean(),
   pending: z.boolean().optional(),
-}) satisfies z.ZodType<Wire<PlanModeState>>
+}).refine(
+  state => state.pending === undefined || state.pending !== state.active,
+  { message: 'pending must differ from active when present', path: ['pending'] },
+) satisfies z.ZodType<Wire<PlanModeState>>
 
 /** session.planMode request payload. */
 export const sessionPlanModeRequestSchema = z.object({

packages/host/apiproxy/src/api/sessions.ts

@@ -47,7 +47,7 @@ export interface SessionSummary {
 /**
  * Plan collaboration state exposed to clients. `active` is the logged state
  * shaping the current request; `pending`, when present, is the user's
- * next-boundary selection.
+ * next-boundary selection and differs from `active`.
  */
 export interface PlanModeState {
   active: boolean