Add state.open_to_visitors (boolean) to distinguish public vs members-only opening

[iooner]

Summary
Add optional state.open_to_visitors: boolean to indicate if the space is currently open to non-members/visitors (public access), as distinct from state.open: boolean which only signals physical open/closed.

Motivation
Today many spaces encode “public vs members-only” only in state.message. A boolean enables machine-readable behavior for clients (websites, signage, aggregators) without string parsing, while keeping message for human text. The docs themselves cite “open for public / members only” as typical messages, which this proposal formalizes.

We (Liège Hackerspace) already use it: https://lghs.be/sapi/

Semantics

• open=true and open_to_visitors=true: open and public (visitor night, event).
• open=true and open_to_visitors=false: open but members-only.
• open=false: closed. open_to_visitors should be omitted or treated as false.
• Field is OPTIONAL. Absence means unknown.

Schema change (v16)

"open_to_visitors": {
  "type": "boolean",
  "description": "Whether the space is currently open to non-members/visitors (public access)."
}

Examples at our space

// Public event
"state": { "open": true, "message": "LiLiT Linux Meeting Party : https://www.lilit.be/index.php?page=contact#LMP", "open_to_visitors": true }

// Weekly visitors night
"state": { "open": true, "message": "Ouvert aux visiteurs", "open_to_visitors": true }

// Members-only opening
"state": { "open": true, "message": "Ouvert aux membres uniquement.", "open_to_visitors": false }

Note: This is just an observation and a suggestion, not a request to integrate it exactly as written. I am not attached to this specific shape; if maintainers prefer a different name, placement, or even a different approach.