feat(client-runtime): project the archive set and the archiveSession action

WorkspaceListState gains archivedSessionIds (ReadonlySet, replaced only
on membership change), installed as full snapshots from the list
baseline, the unary echo, and the changed frame. Archiving the current
session clears the selection into the New Session view state. Test
doubles (test-runtime, fake APIs, fixture client) follow the widened
IWorkspaces/IApiClient faces.

packages/client/runtime/src/client/contract/workspaces.ts

@@ -76,4 +76,11 @@ export interface IWorkspaces {
    * @returns the updated Workspace view.
    */
   insertSessionBefore(workspaceId: WorkspaceId, sessionId: SessionId, beforeSessionId?: SessionId): Promise<WorkspaceView>
+  /**
+   * Archive a session into the registry-global set (hidden from grouping
+   * surfaces; session log and accounting slot remain). Archiving the current
+   * session clears the selection into the New Session view state.
+   * @param sessionId - session to archive.
+   */
+  archiveSession(sessionId: SessionId): Promise<void>
 }

packages/client/runtime/src/client/workspaces/manager.ts

@@ -14,6 +14,8 @@ export type WorkspaceListPhase = 'pending' | 'ready'
 /** Immutable workspace-list snapshot. */
 export interface WorkspaceListSnapshot {
   items: readonly WorkspaceView[]
+  /** Registry-global archive set (hidden from grouping surfaces; accounting slots retained). */
+  archivedSessionIds: ReadonlySet<SessionId>
   state: 'idle' | 'loading' | 'error'
   phase: WorkspaceListPhase
   error: RpcError | null
@@ -28,6 +30,9 @@ export class WorkspaceManager {
   private items: Workspace[] = []
   private itemViewsSource: readonly Workspace[] | null = null
   private itemViewsCache: readonly WorkspaceView[] = []
+  // Full-snapshot state (list response / unary response / changed frame all
+  // carry the complete set), so deltas never merge — installs replace.
+  private archivedSessionIds: ReadonlySet<SessionId> = new Set()
   private state: WorkspaceListSnapshot['state'] = 'idle'
   private phase: WorkspaceListPhase = 'pending'
   private error: RpcError | null = null
@@ -77,6 +82,7 @@ export class WorkspaceManager {
           items = items.filter(workspace => !this.removedIds.has(workspace.workspaceId))
           for (const delta of frames) items = applyWorkspaceDelta(items, delta)
           this.installViews(items)
+          this.installArchived(result.value.archivedSessionIds)
           this.state = 'idle'
           this.phase = 'ready'
         } else {
@@ -158,6 +164,18 @@ export class WorkspaceManager {
     return result
   }
 
+  /**
+   * Archive one session in the registry-global set, then install the
+   * returned full set without waiting for the changed frame.
+   * @param sessionId - session to archive.
+   * @returns the wire result.
+   */
+  async archiveSession(sessionId: SessionId): Promise<RpcResult<{ archivedSessionIds: SessionId[] }>> {
+    const { result } = await this.api.workspace.archiveSession({ sessionId })
+    if (result.ok) this.installArchived(result.value.archivedSessionIds)
+    return result
+  }
+
   /**
    * Host-frame entry. Non-workspace frames are ignored so the runtime can
    * fan one host stream out to both object managers.
@@ -166,6 +184,9 @@ export class WorkspaceManager {
   handleHostEnvelope(envelope: RpcRequest<HostFrame>): void {
     if (envelope.payload.type === 'host/workspace-changed') this.upsert(envelope.payload.workspace)
     else if (envelope.payload.type === 'host/workspace-removed') this.remove(envelope.payload.workspaceId)
+    else if (envelope.payload.type === 'host/archived-sessions-changed') {
+      this.installArchived(envelope.payload.archivedSessionIds)
+    }
   }
 
   /** Re-pull the baseline after each connection generation. */
@@ -194,12 +215,21 @@ export class WorkspaceManager {
   private buildSnapshot(): WorkspaceListSnapshot {
     return {
       items: this.itemViews(),
+      archivedSessionIds: this.archivedSessionIds,
       state: this.state,
       phase: this.phase,
       error: this.error,
     }
   }
 
+  /** Replace the archive set when membership actually changed (set identity backs Object.is short-circuits). */
+  private installArchived(archivedSessionIds: readonly SessionId[]): void {
+    if (archivedSessionIds.length === this.archivedSessionIds.size
+      && archivedSessionIds.every(id => this.archivedSessionIds.has(id))) return
+    this.archivedSessionIds = new Set(archivedSessionIds)
+    this.notifier.markDirty()
+  }
+
   /** Upsert one Host view, optionally retaining the local object that materialized it. */
   private upsert(view: WorkspaceView, identity?: Workspace): void {
     if (this.removedIds.has(view.workspaceId)) return

packages/client/runtime/src/client/workspaces/service.ts

@@ -14,6 +14,12 @@ import { WorkspaceManager, type WorkspaceListPhase } from './manager.ts'
 /** Workspace list plus the two-baseline readiness and default-target projection. */
 export interface WorkspaceListState {
   items: readonly WorkspaceView[]
+  /**
+   * Registry-global archive set: grouping surfaces hide these sessions
+   * everywhere (workspace groups and the ungrouped bucket) while their
+   * session logs and workspace accounting slots remain.
+   */
+  archivedSessionIds: ReadonlySet<SessionId>
   state: 'idle' | 'loading' | 'error'
   phase: WorkspaceListPhase
   error: RpcError | null
@@ -58,7 +64,7 @@ export class WorkspacesService implements IWorkspaces {
   constructor(ctx: Context, private readonly api: IApiClient, private readonly sessions: SessionsPort) {
     this.manager = new WorkspaceManager(api)
     this.list = createSnapshotStore<WorkspaceListState>({
-      items: [], state: 'idle', phase: 'pending', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'pending', error: null,
       baselinesReady: false, recentWorkspaceId: undefined,
     })
     this.manager.subscribe(() => { this.project() })
@@ -249,6 +255,18 @@ export class WorkspacesService implements IWorkspaces {
     if (!result.ok) throw new Error(`workspace delete failed: ${result.error.code}: ${result.error.message}`)
   }
 
+  /**
+   * Archive a session into the registry-global set. When the archived
+   * session is the current one, the selection is cleared into the New
+   * Session view state — a hidden row must not stay open behind the list.
+   * @param sessionId - session to archive.
+   */
+  async archiveSession(sessionId: SessionId): Promise<void> {
+    const result = await this.manager.archiveSession(sessionId)
+    if (!result.ok) throw new Error(`session archive failed: ${result.error.code}: ${result.error.message}`)
+    if (this.sessions.list.getSnapshot().current === sessionId) this.sessions.clear()
+  }
+
   /**
    * Move a session within its Workspace's manual order (DOM-insertBefore-like).
    * @param workspaceId - owning workspace.
@@ -293,6 +311,7 @@ export class WorkspacesService implements IWorkspaces {
     const baselinesReady = workspace.phase === 'ready' && sessions.phase === 'ready'
     this.list.set({
       items: workspace.items,
+      archivedSessionIds: workspace.archivedSessionIds,
       state: workspace.state,
       phase: workspace.phase,
       error: workspace.error,