feat(client-federation): user-view cache for cross-instance DM render

Fixes a render bug where a federated user (e.g. axel@nova) appeared with
the federation globe icon and a broken avatar when viewed on his own home
instance. Root cause: `populateFromReady` is first-wins by federatedId and
discards the entire skipped DM payload — including its `members` array —
so when a sibling instance's ready arrived first, the home instance's view
of every shared user was dropped on the floor.

Adds a render-only `userViews` cache that mirrors the `dmAlternatives`
philosophy: information from skipped ready payloads is preserved for
rendering. Every wire surface that delivers a User upserts into the cache
regardless of dedup outcome; render sites read through a Zustand selector
hook to surface the home view when one is loaded. The DM channel ingestion
race is left untouched — the existing no-flapping invariant on origin
reconnect is intentional and load-bearing for failover.

Layered changes:

- `identity.ts`: `normalizeOriginToHost`, `canonicalUserKey`,
  `isDeliveryFromHome`, `isFederationGlobeApplicable` — single helpers
  for origin/host normalization and the home/stub tier decision.
- `spaceStore.ts`: `userViews` Map, `UserViewEntry` type, `upsertUserView`
  action with the home-wins preference rule, prune by `deliveredBy` in
  `removeInstanceSpaces` (mirrors `dmAlternatives` cleanup), `reset`
  clears.
- `userViewLookup.ts`: `useCanonicalUserView` (Zustand selector hook for
  React) + `getCanonicalUserView` (sync getter for non-React paths).
  Render reactivity is structural via the selector, not coincidence on
  legacy update paths.
- `populateFromReady` upsert pass runs BEFORE the federatedId dedup so
  members of skipped DMs still reach the cache.
- WS handlers (dm_message_*, message_*, user_updated, member_joined,
  friend_request_*, dm_channel_created, dm_member_added) and REST
  hydrators (socialStore, discoverStore, mutuals) feed the cache with
  their delivering origin.
- Render-site routing through `useCanonicalUserView` at every audited
  user-rendering site (sidebar, header, search, message bubble, reply
  chips, profile popout/modal, group settings, voice tiles, mention
  chips, member lists, friends, invites). Self-rendering sites compose
  alongside via existing `isSelf`/`resolveDisplayIdentity`.
- Globe predicate hoisted to `isFederationGlobeApplicable` and applied
  at three sites, gating on `domain !== window.location.host` so we
  never show the globe for users whose home IS our own.

Tests: 31 new unit tests across `identity`, `userViews` store, and
`userViewLookup`. Full suite 276/276.

Docs: `client-federation.md` §3 gains a "User View Cache" section
parallel to "DM Origin Failover"; `dm-system.md` notes the new store
action and WS handler upserts.

Bug 3 (federation profile-sync gap — orbit's stale profile data on
nova-Axel after a clear/color-change on nova never propagated)
remains open. The user-view cache routes around it for the common case
(home instance is connected), but the underlying S2S relay gap is its
own diagnosis and follows in a separate branch.

packages/web/src/components/voice/IncomingCallModal.tsx

@@ -3,7 +3,9 @@ import { useVoiceStore } from '../../stores/voiceStore';
 import { useSpaceStore, getChannelOrigin } from '../../stores/spaceStore';
 import { wsSend } from '../../hooks/useWebSocket';
 import { parseFederatedUsername } from '../../utils/identity';
+import { useCanonicalUserView } from '../../utils/userViewLookup';
 import { Avatar } from '../ui/Avatar';
+import type { User } from '@backspace/shared';
 
 export function IncomingCallModal() {
   const incomingCall = useVoiceStore((s) => s.incomingCall);
@@ -31,11 +33,18 @@ export function IncomingCallModal() {
 
   const dmChannels = useSpaceStore((s) => s.dmChannels);
 
+  const _FALLBACK_USER = { id: '', username: '', createdAt: 0, isAdmin: false, replicatedInstances: [] } as unknown as User;
+  // Look up caller member before the early return so useCanonicalUserView is always called
+  const _rawCallerMember = (() => {
+    if (!incomingCall) return null;
+    const dmChannel = dmChannels.find(d => d.id === incomingCall.dmChannelId);
+    return dmChannel?.members.find(m => m.id === incomingCall.callerId) ?? null;
+  })();
+  const _canonicalCaller = useCanonicalUserView((_rawCallerMember as User | null) ?? _FALLBACK_USER);
+  const callerMember = _rawCallerMember ? _canonicalCaller : null;
+
   if (!incomingCall) return null;
 
-  // Look up the caller in DM channel members for homeUserId
-  const dmChannel = dmChannels.find(d => d.id === incomingCall.dmChannelId);
-  const callerMember = dmChannel?.members.find(m => m.id === incomingCall.callerId);
   const callerAvatarId = callerMember?.homeUserId ?? incomingCall.callerId;
   const { baseName: callerBaseName } = parseFederatedUsername(incomingCall.callerName);