`?online=true` and the `solo` join flag are vestigial

[WilsonNet]

Summary

Two flags survive that no longer decide anything:

• ?online=true used to mean "wait for a human instead of a bot". Rooms are
  addressed by id now, so there is no placement left for it to choose.
• solo on the join message used to pick how a room was filled. Bots are
  opt-in now, so ?bots= and ?fill= decide that instead.

Both are still sent, still parsed, and both feed nothing but the connecting
status line and __gameState().soloMatch.

Why bother

They are actively misleading in the places people read first. ?online=true
appears throughout docs/running-the-game.md and reads as though it enables
something — a new reader reasonably concludes there is an offline-ish default,
when in fact every room is online and always has been. soloMatch in the debug
hook suggests a mode that does not exist.

Suggested approach

• Keep accepting online and solo on the wire so older links keep working —
  people will have pasted room URLs into chats.
• Drop them from the docs and from the mode tables, and remove soloMatch from
  GameStateSnapshot (updating verify-modes.mjs, which prints it).
• Decide what the status line should say instead, since it currently branches on
  solo.

Definition of done

• No document tells a reader to add ?online=true for a behaviour it does not
  produce.
• An old link with ?online=true&room=<id> still lands in the right room.

Notes

Marked vestigial in specs/netcode.md when uuid rooms landed. Filing it so the
cleanup is not lost.