Allow PHOTON_HOME_CHANNEL to accept a bare E.164 phone number or a
`any;-;+1...` DM chat GUID in addition to a Spectrum space id. Inbound
DM spaces are cached so replies resolve without a second SDK lookup,
and `photon` is added to _PHONE_PLATFORMS so send_message treats E.164
strings as explicit targets rather than falling through to channel-name
resolution.
