ipn/ipnlocal,wgengine: move disco-key change detection to nodeBackend

Engine.Reconfig previously diffed cfg.Peers disco keys against the
previous config to find restarted peers and flush their WireGuard
sessions, with a TSMP-learned-key map to suppress resets for key
changes that arrived over a working session. That was the last
per-peer state computed from wgcfg.Config.Peers inside the engine,
and it only ran on full reconfigs, so incremental netmap deltas
never got session resets at all.

Move the detection into nodeBackend, which sees every peer change:
full netmaps in SetNetMap and incremental upserts in
UpdateNetmapDelta both now report which peers changed disco keys,
with the same TSMP suppression and mismatch accounting as before.
LocalBackend acts on the result via a new Engine.ResetDevicePeer
method, which just removes the peer from the WireGuard device and
lets the peer lookup func lazily re-create it with fresh state.

LocalBackend.PatchDiscoKey now records TSMP-learned keys in
nodeBackend instead of forwarding to the engine, so the engine's
PatchDiscoKey method and tsmpLearnedDisco map are gone. The
controlclient patchDiscoKeyer interface becomes the exported
DiscoKeyUpdater so LocalBackend can compile-time assert that it
implements it, alongside its NetmapDeltaUpdater friends, replacing
the test that asserted the same of the engine.

This is one of the last steps toward removing Peers from wgcfg.Config.

Updates #12542

Change-Id: I6b42e460f42924816beae89ca43731cb91b66054
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/auto.go

@@ -575,13 +575,13 @@ func (mrs mapRoutineState) UpdateUserProfiles(profiles map[tailcfg.UserID]tailcf
 	}
 }
 
-var _ patchDiscoKeyer = mapRoutineState{}
+var _ DiscoKeyUpdater = mapRoutineState{}
 
 func (mrs mapRoutineState) PatchDiscoKey(pub key.NodePublic, disco key.DiscoPublic) {
 	c := mrs.c
 	c.mu.Lock()
 	goodState := c.loggedIn && c.inMapPoll
-	dun, ok := c.observer.(patchDiscoKeyer)
+	dun, ok := c.observer.(DiscoKeyUpdater)
 	mapCtx := c.mapCtx
 	c.mu.Unlock()
 

control/controlclient/direct.go

@@ -286,10 +286,10 @@ type UserProfileUpdater interface {
 	UpdateUserProfiles(profiles map[tailcfg.UserID]tailcfg.UserProfileView) bool
 }
 
-// patchDiscoKeyer is an optional interface that can be implemented by an [Observer] to be
+// DiscoKeyUpdater is an optional interface that can be implemented by an [Observer] to be
 // notified about node disco keys received out-of-band from control, via
 // existing connection state.
-type patchDiscoKeyer interface {
+type DiscoKeyUpdater interface {
 	// PatchDiscoKey reports to the receiver that the specified disco key
 	// for node was obtained out-of-band from control.
 	PatchDiscoKey(key.NodePublic, key.DiscoPublic)

control/controlclient/map.go

@@ -362,7 +362,7 @@ func (ms *mapSession) handleNonKeepAliveMapResponse(ctx context.Context, resp *t
 }
 
 func (ms *mapSession) tryMarkDiscoAsLearnedFromTSMP(res *tailcfg.MapResponse) {
-	dun, ok := ms.netmapUpdater.(patchDiscoKeyer)
+	dun, ok := ms.netmapUpdater.(DiscoKeyUpdater)
 	if !ok {
 		return
 	}

control/controlclient/map_test.go

@@ -34,9 +34,7 @@ import (
 	"tailscale.com/util/eventbus/eventbustest"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/must"
-	"tailscale.com/util/usermetric"
 	"tailscale.com/util/zstdframe"
-	"tailscale.com/wgengine"
 )
 
 func eps(s ...string) []netip.AddrPort {
@@ -1949,20 +1947,6 @@ func TestLearnZstdOfKeepAlive(t *testing.T) {
 	}
 }
 
-func TestPathDiscokeyerImplementations(t *testing.T) {
-	bus := eventbustest.NewBus(t)
-	ht := health.NewTracker(bus)
-	reg := new(usermetric.Registry)
-	e, err := wgengine.NewFakeUserspaceEngine(t.Logf, 0, ht, reg, bus)
-	if err != nil {
-		t.Fatal(err)
-	}
-	t.Cleanup(e.Close)
-	if _, ok := e.(patchDiscoKeyer); !ok {
-		t.Error("wgengine.userspaceEngine must implement patchDiscoKeyer")
-	}
-}
-
 func TestPeerIDAndKeyByTailscaleIP(t *testing.T) {
 	peerKey1 := key.NewNode().Public()
 	peerKey2 := key.NewNode().Public()