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/projection-store.client.spec.ts

@@ -209,7 +209,7 @@ describe('manager frame routing', () => {
     expect(manager.getListSnapshot().items[0]?.projectionValues).not.toBe(baseline)
   })
 
-  it('drops the projection store with the removed session', async () => {
+  it('retains the projection store with the removed session (the instance survives removal)', async () => {
     const api = new FakeApiClient()
     const manager = new SessionManager(api, fakeRemote())
     api.onList = () => Promise.resolve(ok({
@@ -224,6 +224,9 @@ describe('manager frame routing', () => {
       rpcId: 'rm' as never,
       payload: { type: 'host/session-removed', sessionId: sid('s1') } as never,
     })
-    expect(manager.get(sid('s1')).projections.get('title')).toBeUndefined()
+    // The Session instance survives removal (resident-instance rule), so its
+    // projection face survives with it: a re-listed session resumes its
+    // values instead of re-seeding from an empty face.
+    expect(manager.get(sid('s1')).projections.get('title')).toBe('Doomed')
   })
 })