ipn/ipnlocal, control/controlclient: process node adds/removes in constant time

For large tailnets (~50k+ nodes) with frequent peer churn (ephemeral
GitHub Actions workers etc.), tailscaled used to rebuild the full
netmap and fan it out on the IPN bus on every MapResponse that
added or removed a peer. There were two O(N) costs per delta: the
full netmap rebuild + every Notify.NetMap encode to every bus watcher.

This change tackles both:

  1. Plumb O(1) peer add/remove through the delta path. PeersChanged
     and PeersRemoved no longer prevent the delta happy path; instead,
     they mutate the per-node-backend peer map in place.

  2. Restrict ipn.Notify.NetMap emission to the platforms whose host
     GUIs still depend on it (Windows, macOS, iOS) and migrate
     in-tree consumers off it everywhere else:

     - Migrate reactive consumers (containerboot, kube agents,
       sniproxy, tsconsensus, etc.) off Notify.NetMap to the
       previously-added Notify.SelfChange signal so they no longer
       have to subscribe to the full netmap.
     - Add ipn.NotifyNoNetMap so GUI clients on "legacy-emit" platforms
       that have already migrated can opt out of the per-watcher
       NetMap encode.
     - Gate Notify.NetMap emission on the producer side by a compile-
       time GOOS check, so the supporting code is dead-code-eliminated
       on Linux and other geese where no GUI consumer needs it.

Re-running BenchmarkGiantTailnet from tstest/largetailnet, which was
added along with baseline numbers on unmodified main in ad5436af0d,
the per-delta cost (one peer add+remove pair) is now ~O(1) regardless
of tailnet size N:

    N         no-watcher (ms/op)            bus-watcher (ms/op)
              before    now     factor      before    now     factor
     10000        32   0.11       300x         166   0.13      1300x
     50000       222   0.11      2000x         865   0.13      6700x
    100000       504   0.12      4100x        1765   0.13     13400x
    250000      1551   0.12     12500x        4696   0.15     32400x

Updates #12542

Change-Id: I94e34b37331d1a8ec74c299deffadf4d061fda9e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/local/local.go

@@ -1094,11 +1094,13 @@ func (lc *Client) DNSConfig(ctx context.Context) (*tailcfg.DNSConfig, error) {
 }
 
 // PeerByID returns a peer's current full [tailcfg.Node] looked up by its
-// [tailcfg.NodeID], in O(1) time on the daemon side. It returns an error
-// if no peer with that NodeID is in the current netmap.
+// [tailcfg.NodeID]. It returns an error if no peer with that NodeID is in the
+// current netmap.
 //
-// It is intended for callers that need the latest state of a single peer
-// without fetching the entire netmap.
+// It is intended for callers that observed a peer-mutation signal (e.g.
+// [ipn.Notify.PeerChangedPatch] or [ipn.Notify.PeersChanged]) and want the
+// latest state of the affected node without having to apply the patch
+// themselves.
 func (lc *Client) PeerByID(ctx context.Context, id tailcfg.NodeID) (*tailcfg.Node, error) {
 	body, err := lc.get200(ctx, "/localapi/v0/peer-by-id?id="+strconv.FormatInt(int64(id), 10))
 	if err != nil {
@@ -1107,6 +1109,22 @@ func (lc *Client) PeerByID(ctx context.Context, id tailcfg.NodeID) (*tailcfg.Nod
 	return decodeJSON[*tailcfg.Node](body)
 }
 
+// UserProfile returns the current [tailcfg.UserProfile] for the given
+// [tailcfg.UserID]. It returns an error if no user with that UserID is in the
+// current netmap.
+//
+// It is the LocalAPI fallback for IPN-bus consumers that see a UserID
+// referenced by a peer Node and want to resolve it to a UserProfile. Sessions
+// opted in to [ipn.NotifyPeerChanges] / [ipn.NotifyPeerPatches] also receive
+// UserProfiles automatically via [ipn.Notify.UserProfiles].
+func (lc *Client) UserProfile(ctx context.Context, id tailcfg.UserID) (*tailcfg.UserProfile, error) {
+	body, err := lc.get200(ctx, "/localapi/v0/user-profile?id="+strconv.FormatInt(int64(id), 10))
+	if err != nil {
+		return nil, err
+	}
+	return decodeJSON[*tailcfg.UserProfile](body)
+}
+
 // PingOpts contains options for the ping request.
 //
 // The zero value is valid, which means to use defaults.