fix: keep client session projection stores across removal and revival

The per-session ProjectionValueStore was deleted on host/session-removed
even though the Session instance survives removal (open() is idempotent and
drop() only removes the map entry). A re-added or re-listed session resumed
on the same instance with an empty projection face, so projection-backed UI
(such as the OpenRouter cost readout) reverted to blank until a fresh host
baseline landed.

Retain the store on removal and lift the removed tombstone from the
surviving instance on revival (host/session-added and list-refresh
re-listing), so chat switching keeps cost readouts populated.

packages/client/runtime/tests/switch-retention.client.spec.ts

@@ -0,0 +1,147 @@
+/**
+ * Repro + behavioral pin for the chat-switch projection retention contract:
+ * a session's projection store (openRouterCost is the canonical example) must
+ * survive a selection change and remain readable through the same value face
+ * when the session becomes current again.
+ *
+ * Server frames exercised:
+ * - session.list rows carrying a projections block (the cold seed path)
+ * - session/projection push frames to a live session
+ * - host/session-removed (transient host teardown) + host/session-added
+ * - session/subscribed (durable-replay baseline) + a re-pulled history tail
+ *
+ * The manager's per-session ProjectionValueStore is retained across select();
+ * this suite pins the retention contract so a regression in the
+ * retention/seeding discipline shows up as a red test here.
+ */
+import { describe, expect, it } from 'vitest'
+import type { SessionId } from '@deepseek-ai/dsh-api-remotes/client'
+import { SessionManager } from '../src/client/sessions/manager.ts'
+import { FakeApiClient, fakeRemote, ok } from './fake-api.client.ts'
+
+const S1 = 'fk-s1' as SessionId
+const S2 = 'fk-s2' as SessionId
+
+const COST = { totalUsd: 1.23, pricedSteps: 1, unknownModelSteps: 0, currency: 'USD' as const }
+
+function summary(sessionId: SessionId, projections?: { asOfSeq: number; values: Record<string, unknown> }) {
+  return {
+    sessionId,
+    updatedAt: 100,
+    running: false,
+    blank: false,
+    ...(projections === undefined ? {} : { projections }),
+  } as never
+}
+
+describe('projection retention across chat switches (repro)', () => {
+  it('retains the projection store for a session that was opened, seeded, and switched away from and back', async () => {
+    const api = new FakeApiClient()
+    api.onList = () => Promise.resolve(ok({ items: [summary(S1), summary(S2)] }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+
+    // A live push frame lands the cost in S1's resident store.
+    manager.handleMuxEnvelope({
+      rpcId: 'seed' as never,
+      payload: { type: 'session/projection', sessionId: S1, key: 'openRouterCost', value: COST, seq: 5 } as never,
+    })
+    expect(manager.getListSnapshot().items[0]?.projectionValues?.openRouterCost).toEqual(COST)
+
+    // The conversation switches to another chat and back; nothing removes S1.
+    manager.select(S2)
+    manager.select(S1)
+    expect(manager.getListSnapshot().items[0]?.projectionValues?.openRouterCost).toEqual(COST)
+  })
+
+  it('a host/session-removed disposal followed by a re-add re-seeds the cost from the list row block', async () => {
+    const api = new FakeApiClient()
+    api.onList = () => Promise.resolve(ok({
+      items: [
+        summary(S1, { asOfSeq: 6, values: { openRouterCost: COST } }),
+        summary(S2),
+      ],
+    }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)?.projectionValues?.openRouterCost)
+      .toEqual(COST)
+
+    // Transient host teardown: the chat leaves the list; its projection store
+    // is retained (the Session instance survives removal), and the durable
+    // chat still exists host-side, so a list refresh re-adds it.
+    manager.handleHostEnvelope({
+      rpcId: 'rm' as never,
+      payload: { type: 'host/session-removed', sessionId: S1 } as never,
+    })
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)).toBeUndefined()
+
+    await manager.refreshList()
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)?.projectionValues?.openRouterCost)
+      .toEqual(COST)
+  })
+
+  it('lifts the removed tombstone when a disposed session is re-listed by the host', async () => {
+    const api = new FakeApiClient()
+    api.onList = () => Promise.resolve(ok({ items: [summary(S1), summary(S2)] }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+
+    const session = manager.get(S1)
+    manager.handleHostEnvelope({
+      rpcId: 'rm' as never,
+      payload: { type: 'host/session-removed', sessionId: S1 } as never,
+    })
+    expect(session.getSnapshot().removed).toBe(true)
+    // The same instance survives (resident-instance rule).
+    expect(manager.get(S1)).toBe(session)
+
+    // The durable chat comes back: the re-listing revives the surviving
+    // instance rather than leaving the conversation tombstoned forever.
+    manager.handleHostEnvelope({
+      rpcId: 'add' as never,
+      payload: { type: 'host/session-added', blank: false, sessionId: S1 } as never,
+    })
+    expect(session.getSnapshot().removed).toBe(false)
+  })
+
+  it('revives through a list refresh when the host re-lists a previously disposed session', async () => {
+    const api = new FakeApiClient()
+    const listed = [summary(S1), summary(S2)]
+    api.onList = () => Promise.resolve(ok({ items: listed }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+
+    const session = manager.get(S1)
+    manager.handleHostEnvelope({
+      rpcId: 'rm' as never,
+      payload: { type: 'host/session-removed', sessionId: S1 } as never,
+    })
+    expect(session.getSnapshot().removed).toBe(true)
+
+    // The host never re-emits host/session-added for an existing id; the
+    // durable session simply appears again on the next list pull.
+    await manager.refreshList()
+    expect(session.getSnapshot().removed).toBe(false)
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)).toBeDefined()
+  })
+
+  it('survives a subscribed durable-replay baseline when the value is at or before the baseline', async () => {
+    const api = new FakeApiClient()
+    api.onList = () => Promise.resolve(ok({ items: [summary(S1)] }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+
+    manager.handleMuxEnvelope({
+      rpcId: 'push' as never,
+      payload: { type: 'session/projection', sessionId: S1, key: 'openRouterCost', value: COST, seq: 3 } as never,
+    })
+
+    // The durable baseline is at seq 3, so the row is not phantom: it survives.
+    manager.handleMuxEnvelope({
+      rpcId: 'sub' as never,
+      payload: { type: 'session/subscribed', sessionId: S1, lastSeq: 3 } as never,
+    })
+    expect(manager.getListSnapshot().items[0]?.projectionValues?.openRouterCost).toEqual(COST)
+  })
+})