net/tstun,wgengine,ipn/ipnlocal: make tstun's peerConfigTable use RouteManager table

Previously tstun.Wrapper.SetWGConfig walked wgcfg.Config.Peers on every
netmap to rebuild its own IP-to-peer table for masquerade NAT rewrites
and jailed-peer classification. Now the tun layer instead consumes the
route manager's shared immutable outbound snapshot directly, via a new
Engine.SetPeerRoutes method: LocalBackend pushes the snapshot (plus this
node's native Tailscale addresses) after every route manager commit that
can change it, and per-packet lookups read the interned PeerRoute
attributes from that table.

When no current peer is jailed or masqueraded, LocalBackend installs a
nil table (gated on RouteManager.HasDataPlaneAttrs), preserving the
per-packet nil-check fast path. The exitNodeRequiresMasq machinery is
deleted: its purpose was populating the table with all peers so that
more-specific entries shadow an exit node's /0, and the always-full
route manager table gives that shadowing inherently.

This is the last step before removing the Peers field from wgcfg.Config.

Updates #12542

Change-Id: Ifce09ca929a3f2511303ca1d6efdd583739494ce
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tsaddr/tsaddr.go

@@ -7,6 +7,7 @@ package tsaddr
 import (
 	"encoding/binary"
 	"errors"
+	"iter"
 	"net/netip"
 	"slices"
 	"sync"
@@ -317,3 +318,24 @@ func MapVia(siteID uint32, v4 netip.Prefix) (via netip.Prefix, err error) {
 	copy(a[12:], ip4a[:])
 	return netip.PrefixFrom(netip.AddrFrom16(a), v4.Bits()+64+32), nil
 }
+
+// FirstTailscaleAddrs returns the first Tailscale IPv4 address and
+// the first Tailscale IPv6 address among the addresses of addrs'
+// prefixes, if any. The addrs sequence is typically a node's own
+// address list, either a slice (via [slices.All]) or a view (via
+// [views.Slice.All]).
+func FirstTailscaleAddrs(addrs iter.Seq2[int, netip.Prefix]) (a4, a6 netip.Addr) {
+	for _, pfx := range addrs {
+		a := pfx.Addr()
+		switch {
+		case a.Is4() && !a4.IsValid() && IsTailscaleIP(a):
+			a4 = a
+		case a.Is6() && !a6.IsValid() && IsTailscaleIP(a):
+			a6 = a
+		}
+		if a4.IsValid() && a6.IsValid() {
+			break
+		}
+	}
+	return a4, a6
+}