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/modals/NewDmModal.tsx

@@ -7,6 +7,33 @@ import { useSpaceStore } from '../../stores/spaceStore';
 import { api } from '../../api/client';
 import type { User } from '@backspace/shared';
 import { parseFederatedUsername } from '../../utils/identity';
+import { useCanonicalUserView } from '../../utils/userViewLookup';
+
+function NewDmUserRow({
+  user,
+  onSelect,
+}: {
+  user: User;
+  onSelect: (user: User) => void;
+}) {
+  const canonical = useCanonicalUserView(user);
+  const { baseName } = parseFederatedUsername(canonical.username);
+  const displayName = canonical.displayName ?? baseName;
+  return (
+    <button
+      onClick={() => onSelect(user)}
+      className="w-full flex items-center gap-3 px-3 py-2 rounded-[4px] hover:bg-interactive-hover transition-colors text-left"
+    >
+      <Avatar src={canonical.avatar} name={displayName} size={36} status={canonical.status as any} userId={canonical.homeUserId ?? canonical.id} avatarColor={canonical.avatarColor} />
+      <div className="flex-1 min-w-0">
+        <div className="text-[14px] font-medium text-txt-primary truncate">
+          {displayName}
+        </div>
+        <div className="text-[12px] text-txt-tertiary truncate">@{canonical.username}</div>
+      </div>
+    </button>
+  );
+}
 
 export function NewDmModal() {
   const [query, setQuery] = useState('');
@@ -106,25 +133,13 @@ export function NewDmModal() {
             <div className="py-4 text-center text-txt-tertiary text-[14px]">No users found</div>
           )}
 
-          {results.map((user) => {
-            const { baseName } = parseFederatedUsername(user.username);
-            const displayName = user.displayName ?? baseName;
-            return (
-              <button
-                key={user.id}
-                onClick={() => handleSelectUser(user)}
-                className="w-full flex items-center gap-3 px-3 py-2 rounded-[4px] hover:bg-interactive-hover transition-colors text-left"
-              >
-                <Avatar src={user.avatar} name={displayName} size={36} status={user.status as any} userId={user.homeUserId ?? user.id} avatarColor={user.avatarColor} />
-                <div className="flex-1 min-w-0">
-                  <div className="text-[14px] font-medium text-txt-primary truncate">
-                    {displayName}
-                  </div>
-                  <div className="text-[12px] text-txt-tertiary truncate">@{user.username}</div>
-                </div>
-              </button>
-            );
-          })}
+          {results.map((user) => (
+            <NewDmUserRow
+              key={user.id}
+              user={user}
+              onSelect={handleSelectUser}
+            />
+          ))}
         </div>
       </div>
     </Modal>