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/platform_windows.go

@@ -33,6 +33,12 @@ const network = "pipe"
 // exists so a Server is built the same way on every platform.
 var currentUID = func() uint32 { return 0 }
 
+// infoSupported is false here. A pipe has no mode to open to other accounts, and the
+// security descriptor listen builds admits the daemon's account alone, which is the
+// opposite of what the info API is for. Serving it on windows means designing which
+// accounts a pipe admits, and that has not been done.
+const infoSupported = false
+
 // listen creates the named pipe at path with a security descriptor that admits the user
 // nebula runs as, SYSTEM and Administrators, and the group acc names, if any, for
 // reading and writing, which is what connecting to a pipe takes. acc's mode means
@@ -142,14 +148,18 @@ func sddlFor(acc Access) (string, error) {
 const pipeClientAccess uint32 = windows.FILE_READ_DATA | windows.FILE_WRITE_DATA |
 	windows.FILE_READ_ATTRIBUTES | windows.READ_CONTROL | windows.SYNCHRONIZE
 
+// removeSocket is a no-op: a named pipe disappears when its last handle closes, so
+// there is nothing to unlink and no stale instance to clean up.
+func removeSocket(path string, id socketID) {}
+
+// identifySocket has no node to identify: a pipe is its handle, and it cannot be
+// removed from under the daemon, so an endpoint on its path is always still there.
+func identifySocket(path string) (socketID, bool) { return socketID{}, true }
+
 // applyAccess cannot change a pipe's access in place: its security descriptor is fixed
 // when it is created, so a change means creating it again, which the caller does.
 func applyAccess(path string, acc Access) error { return errReopenForAccess }
 
-// removeSocket is a no-op: a named pipe disappears when its last handle closes, so
-// there is nothing to unlink and no stale instance to clean up.
-func removeSocket(path string) {}
-
 // dial connects to the pipe and, before the connection carries anything, checks who
 // serves it. The request names a command and its arguments, so it must not reach a
 // pipe that some other account put under nebula's name. For the default name the