feat(workspace): add registry-global archivedSessionIds set

The workspace domain global singleton gains archivedSessionIds (schema
default [], version unchanged): a display-layer archive set layered over
workspace accounting. archiveSession() rides the registry operation
chain, validates the session against live/persisted headers, and is
idempotent; archived sessions keep their sessionIds slot so a future
unarchive restores position.

packages/workspace/workspace/tests/workspace.spec.ts

@@ -135,9 +135,16 @@ function record(path: string, sessionIds: string[], createdAt = '2026-07-24T00:0
   }
 }
 
+/**
+ * Media written before archivedSessionIds existed omit the field; keeping the
+ * fixtures in that shape continuously proves the schema default upgrades them.
+ */
+type StoredDomainState = Omit<WorkspaceDomainState, 'archivedSessionIds'>
+  & Partial<Pick<WorkspaceDomainState, 'archivedSessionIds'>>
+
 function storedPool(
   entries: Array<[string, WorkspaceRecord]>,
-  state: WorkspaceDomainState,
+  state: StoredDomainState,
 ): MemoryMediaPool {
   const pool = new MemoryMediaPool()
   pool.versions.set('workspace', DOMAIN_VERSION)
@@ -185,7 +192,7 @@ describe('WorkspaceRegistry lifecycle and bootstrap', () => {
     await fiber.await()
     expect(ctx.workspace.list()).toEqual([])
     expect(list).toHaveBeenCalledTimes(1)
-    expect(storedState(pool)).toEqual({ initialized: true, workspaceIds: [] })
+    expect(storedState(pool)).toEqual({ initialized: true, workspaceIds: [], archivedSessionIds: [] })
   })
 
   it('bootstraps once from list headers only, in workspace/session createdAt order', async () => {
@@ -218,6 +225,7 @@ describe('WorkspaceRegistry lifecycle and bootstrap', () => {
     expect(storedState(result.pool)).toEqual({
       initialized: true,
       workspaceIds: result.registry.list().map(workspace => workspace.id),
+      archivedSessionIds: [],
     })
   })
 
@@ -246,7 +254,7 @@ describe('WorkspaceRegistry lifecycle and bootstrap', () => {
     const second = await harness({ pool, sessions: [header('late', late, 100)] })
     expect(second.list).not.toHaveBeenCalled()
     expect(second.registry.list()).toEqual([])
-    expect(storedState(pool)).toEqual({ initialized: true, workspaceIds: [] })
+    expect(storedState(pool)).toEqual({ initialized: true, workspaceIds: [], archivedSessionIds: [] })
   })
 
   it('reuses partial records after a bootstrap record write fails', async () => {
@@ -476,7 +484,7 @@ describe('WorkspaceRegistry create and lookup', () => {
     await expect(result.registry.delete(workspace.id)).resolves.toBe(false)
     expect(result.registry.get(workspace.id)).toBeUndefined()
     expect(result.registry.list()).toEqual([])
-    expect(storedState(result.pool)).toEqual({ initialized: true, workspaceIds: [] })
+    expect(storedState(result.pool)).toEqual({ initialized: true, workspaceIds: [], archivedSessionIds: [] })
     expect(result.pool.media.get('workspace')!.tables.get('workspaces')!.has(workspace.id)).toBe(false)
     await expect(realpath(dir)).resolves.toBe(dir)
     expect(result.list).toHaveBeenCalledTimes(1)
@@ -519,6 +527,7 @@ describe('WorkspaceRegistry create and lookup', () => {
     expect(storedState(pool)).toEqual({
       initialized: true,
       workspaceIds: [],
+      archivedSessionIds: [],
       pendingMutation: { operation: 'delete', workspaceId: workspace.id },
     })
     const reregistered = await first.registry.create(dir)
@@ -526,6 +535,7 @@ describe('WorkspaceRegistry create and lookup', () => {
     expect(storedState(pool)).toEqual({
       initialized: true,
       workspaceIds: [reregistered.id],
+      archivedSessionIds: [],
     })
     await first.fiber.dispose()
 
@@ -762,7 +772,7 @@ describe('header-validated membership projection', () => {
     const createRecovery = await harness({ pool: interruptedCreate })
     expect(createRecovery.registry.list()).toEqual([])
     expect(interruptedCreate.media.get('workspace')!.tables.get('workspaces')!.has(createId)).toBe(false)
-    expect(storedState(interruptedCreate)).toEqual({ initialized: true, workspaceIds: [] })
+    expect(storedState(interruptedCreate)).toEqual({ initialized: true, workspaceIds: [], archivedSessionIds: [] })
 
     const interruptedDelete = storedPool(
       [[deleteId, record(deleteDir, [])]],
@@ -775,7 +785,7 @@ describe('header-validated membership projection', () => {
     const deleteRecovery = await harness({ pool: interruptedDelete })
     expect(deleteRecovery.registry.list()).toEqual([])
     expect(interruptedDelete.media.get('workspace')!.tables.get('workspaces')!.has(deleteId)).toBe(false)
-    expect(storedState(interruptedDelete)).toEqual({ initialized: true, workspaceIds: [] })
+    expect(storedState(interruptedDelete)).toEqual({ initialized: true, workspaceIds: [], archivedSessionIds: [] })
 
     const corruptPending = storedPool(
       [[deleteId, record(deleteDir, [])]],
@@ -816,3 +826,64 @@ describe('workspace mutation and status', () => {
     expect(registry.get(workspace.id)).toBe(workspace)
   })
 })
+
+describe('registry-global session archive', () => {
+  it('archives durably in order, idempotently skips repeats, and leaves accounting untouched', async () => {
+    const dir = await makeDir('archive-home')
+    const result = await harness({ sessions: [header('kept', dir, 100), header('gone', dir, 200)] })
+    const workspace = result.registry.list()[0]!
+    expect(result.registry.archivedSessionIds).toEqual([])
+
+    await result.registry.archiveSession(SessionId('gone'))
+    expect(result.registry.archivedSessionIds).toEqual(['gone'])
+    // Archiving is a display-set write: the workspace account keeps the id.
+    expect(workspace.sessionIds).toContain('gone')
+    expect(storedState(result.pool).archivedSessionIds).toEqual(['gone'])
+    const changesAfterFirst = result.changes.filter(change => change.table === '').length
+
+    await result.registry.archiveSession(SessionId('gone'))
+    expect(result.registry.archivedSessionIds).toEqual(['gone'])
+    // The idempotent repeat neither rewrites the medium nor emits a change.
+    expect(result.changes.filter(change => change.table === '').length).toBe(changesAfterFirst)
+
+    await result.registry.archiveSession(SessionId('kept'))
+    expect(result.registry.archivedSessionIds).toEqual(['gone', 'kept'])
+  })
+
+  it('accepts unaccounted and live sessions but rejects unknown ids without writing', async () => {
+    const dir = await makeDir('archive-strays')
+    const live = await makeDir('archive-live')
+    const result = await harness({
+      sessions: [header('stray', dir, 100)],
+      liveSessions: [header('live-only', live, 200)],
+    })
+    await result.registry.archiveSession(SessionId('stray'))
+    await result.registry.archiveSession(SessionId('live-only'))
+    expect(result.registry.archivedSessionIds).toEqual(['stray', 'live-only'])
+
+    await expect(result.registry.archiveSession(SessionId('ghost')))
+      .rejects.toThrow(/cannot archive session 'ghost'/)
+    expect(storedState(result.pool).archivedSessionIds).toEqual(['stray', 'live-only'])
+  })
+
+  it('restores the archive set across restarts and defaults it for pre-field media', async () => {
+    const dir = await makeDir('archive-restart')
+    const pool = new MemoryMediaPool()
+    const first = await harness({ pool, sessions: [header('s1', dir, 100)] })
+    await first.registry.archiveSession(SessionId('s1'))
+    await first.fiber.dispose()
+
+    const second = await harness({ pool, sessions: [header('s1', dir, 100)] })
+    expect(second.registry.archivedSessionIds).toEqual(['s1'])
+    await second.fiber.dispose()
+
+    // A medium written before the field existed parses through the schema default.
+    const legacyId = WorkspaceId('00000000-0000-4000-8000-00000000000a')
+    const legacy = storedPool(
+      [[legacyId, record(dir, [])]],
+      { initialized: true, workspaceIds: [legacyId] },
+    )
+    const upgraded = await harness({ pool: legacy })
+    expect(upgraded.registry.archivedSessionIds).toEqual([])
+  })
+})