Multiuser support: collaborative field layout

Follow-up to the MVP (#1), which is single-user.

## Feature

Let two or more people lay out the same field simultaneously:

- One user places the field; others join the session and see the identical placement.
- Each participant is guided to a different cone (claim/assign cones), with live progress shared across devices (cone placed / in progress / remaining).
- Roughly halves (or better) the time to set up a field.

## Notes

- Candidate transports: shared session via a lightweight backend, CloudKit sharing, or peer-to-peer (MultipeerConnectivity) for offline fields.
- A field placement is tiny (anchor coordinate + rotation + spec id), so sync payloads are trivial; the interesting part is session join UX (QR code / share link) and cone assignment.

Start with the MVP described in issue #1 and map how a field placement and cone progress could be shared across devices. Compare the listed lightweight backend, CloudKit sharing, and MultipeerConnectivity options, then define the session join flow and cone assignment rules. Done means multiple participants can join one field, see identical placement, and observe shared progress.

Written by the indexing model from the issue text.