Add the identity command and serve it on an opt-in HTTP info API

Build slackhq/nebula#1821 ("who is this vpn address?" over a local
HTTP+JSON socket) on the command registry, so the lookup is one command
on every surface and the socket is one more local API.

identity
- `identity [-address <vpn addr>[:port]]`: no address reports this node;
  with one, this node or the peer behind an established tunnel
- Certificate fields only: name, vpn addresses, networks, unsafe
  networks, groups, fingerprint, issuer, validity, cert version; a peer's
  fingerprint is the one nebula computed when it verified the cert
- Port accepted and stripped, 4in6 unmapped, so a RemoteAddr passes
  straight through
- No tunnel is a commands.NotFound; an empty -address is refused
- Lighthouse TXT lookup shares the owner and peer lookups, keeps answering
  with the initiating certificate, and no longer panics without a default cert
- Served on ssh and nebula ctl too

Info API
- Off until `local_api.info.enabled` with `socket` and optional quoted
  octal `socket_mode` (default "0600"); nothing derived
- Serves commands declaring Command.Identity plus help; others 403 naming
  their surfaces. TestInfoAPISurface pins the set to identity
- GET /v1 is the schema; GET /v1/<cmd>?<flag>=<value> runs one; body is
  the -json rendering via commands.Render; commands.PresentationFormat
  reads json/pretty from the query the way a line is read
- 400 commands.ParamError, 403 commands.WithheldError (what Resolve now
  returns for a withheld name, so help and dispatch type-check it too),
  404 NotFound or unknown, 500 Run error, refusal with no reason, or
  recovered panic with no detail; no redirects; every miss is JSON
- listen takes a mode: bind under umask 0077 then chmod; "0660" plus a
  searchable dir opens it to a group
- The chmod, at bind and on a mode-only reload, goes through chmodSocket,
  which never follows a link: a user who can write the socket's directory
  could otherwise swap the node for a link and have a root daemon put
  0660 or 0666 on any file. Only a socket we own is chmoded. On linux the
  node is held open O_PATH|O_NOFOLLOW, checked by fstat, and chmoded
  through /proc/self/fd, so the check and the chmod land on one inode;
  fchmodat2 is the fallback without /proc. On darwin and the BSDs it is
  an Lstat check and fchmodat(AT_SYMLINK_NOFOLLOW). clearStale Lstats
  too, so a dangling link is refused rather than followed by bind on
  darwin
- One endpoint type owns bind under mutex, bind-before-release move,
  context watch and stop/unlink; each server supplies what it serves
- Reloadable through the same reconcile shape as the control socket:
  mode applied in place, unparsable section keeps what runs, control
  socket path refused by name
- Wire returns Endpoints{Control, Info} with one Stop
- Windows unsupported for now (a pipe has no mode); warned once

Tests: identity lookups and shapes; API filter, status codes, panic
recovery, socket mode, rebind, cancellation; config parsing incl. bare
0660; Wire transitions; e2e of two nebulas with the info API resolving
the peer by address and port, through shared test.HTTPDo/HTTPGet helpers.
Goldens gain identity.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

dns_server_test.go

@@ -2,6 +2,7 @@ package nebula
 
 import (
 	"context"
+	"encoding/json"
 	"log/slog"
 	"net"
 	"net/netip"
@@ -402,6 +403,32 @@ func TestDnsServer_QueryCert_returnsOwnCert(t *testing.T) {
 	assert.Empty(t, ds.QueryCert(other.String()+"."), "unknown peer IP should return nothing")
 }
 
+// TestDnsServer_QueryCert_answersWithTheInitiatingCertificate: with both versions loaded
+// the TXT record carries the certificate handshakes start with, as it always has, where
+// identity and print-cert report the v2.
+func TestDnsServer_QueryCert_answersWithTheInitiatingCertificate(t *testing.T) {
+	ds, _ := newTestDnsServer(t)
+	me := netip.MustParseAddr("10.0.0.1")
+	network := netip.PrefixFrom(me, 24)
+	ca1, _, caKey1, _ := cert_test.NewTestCaCert(cert.Version1, cert.Curve_CURVE25519, time.Time{}, time.Time{}, nil, nil, nil)
+	v1, _, _, _ := cert_test.NewTestCert(cert.Version1, cert.Curve_CURVE25519, ca1, caKey1, "lighthouse", time.Time{}, time.Time{}, []netip.Prefix{network}, nil, nil)
+	ca2, _, caKey2, _ := cert_test.NewTestCaCert(cert.Version2, cert.Curve_CURVE25519, time.Time{}, time.Time{}, nil, nil, nil)
+	v2, _, _, _ := cert_test.NewTestCert(cert.Version2, cert.Curve_CURVE25519, ca2, caKey2, "lighthouse", time.Time{}, time.Time{}, []netip.Prefix{network}, nil, nil)
+
+	addrsTable := new(bart.Lite)
+	addrsTable.Insert(netip.PrefixFrom(me, me.BitLen()))
+	cs := &CertState{v1Cert: v1, v2Cert: v2, initiatingVersion: cert.Version1, myVpnAddrs: []netip.Addr{me}, myVpnAddrsTable: addrsTable}
+	ds.pki = &PKI{}
+	ds.pki.cs.Store(cs)
+
+	var got struct {
+		Version cert.Version `json:"version"`
+	}
+	require.NoError(t, json.Unmarshal([]byte(ds.QueryCert(me.String()+".")), &got))
+	assert.Equal(t, cert.Version1, got.Version)
+	assert.Equal(t, v2, ownCertificate(cs), "identity reports the v2 from the same state")
+}
+
 func TestDnsServer_reload_disable_stopsRunningServer(t *testing.T) {
 	port := freeUDPPort(t)
 	ds, c := newTestDnsServer(t)