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>

localapi/server_test.go

@@ -274,6 +274,38 @@ func TestStopDetachesTheWatcher(t *testing.T) {
 	require.Eventually(t, func() bool { return s.Path() == "" }, time.Second, 10*time.Millisecond, "the second run's context stops it")
 }
 
+// TestGivingUpTheSocketDetachesTheWatcher: when the node is gone from under the endpoint
+// and the bind that follows fails, the run is over and its watcher went with it, so a
+// later Rebind is watched on the ctx it was given, as after a Stop.
+func TestGivingUpTheSocketDetachesTheWatcher(t *testing.T) {
+	s := NewServer(test.NewLogger(), basicRegistry())
+	path := testSocket(t)
+
+	ctxA, cancelA := context.WithCancel(context.Background())
+	t.Cleanup(cancelA)
+	require.NoError(t, s.Rebind(ctxA, path, ownerOnly()))
+
+	// The node goes and a directory takes its place, so the bind that follows fails.
+	require.NoError(t, os.Remove(path))
+	require.NoError(t, os.Mkdir(path, 0700))
+	err := s.Rebind(ctxA, path, ownerOnly())
+	require.ErrorContains(t, err, "is not a socket")
+	assert.Empty(t, s.Path(), "the socket was given up before the bind failed")
+	require.NoError(t, os.Remove(path))
+
+	ctxB, cancelB := context.WithCancel(context.Background())
+	t.Cleanup(cancelB)
+	require.NoError(t, s.Rebind(ctxB, path, ownerOnly()))
+	t.Cleanup(s.Stop)
+
+	cancelA()
+	time.Sleep(50 * time.Millisecond)
+	assert.Equal(t, path, s.Path(), "the first run's context has nothing to do with the second run")
+
+	cancelB()
+	require.Eventually(t, func() bool { return s.Path() == "" }, time.Second, 10*time.Millisecond, "the second run's context stops it")
+}
+
 func TestVersionMismatchIsExplained(t *testing.T) {
 	_, path := serve(t, basicRegistry())