People working together: one session, many machines #9

Several people working in one fizzy session from different machines: each with their own view, their own cursor and their own undo, on the same documents. Agents join the same way.

A stub, so it has a home. The plan comes later. It builds on the rooms from roadmap#8 (plans/AGENTS_ANYWHERE_PLAN.md, the edge-hosted room server), and on what fizzy already has:

• owners report a document's steps and apply recorded ones (sdk: owners report a document's steps, and apply recorded ones fizzy#456), which tapes and tracks are built on;
• authorship per step (plans/SHARED_HISTORY_PLAN.md, #14);
• a view of a document apart from the panes (plans/AGENT_VIEW_PLAN.md, #18).

To design:

• a room-ordered step log, and how an owner's steps travel (the binary step form from tape: steps of a document's history, and a deflated binary form fizzy#453);
• concurrent steps: rebasing, or regional claims, per owner;
• presence: cursors, selections, each person's view, who is acting (the bubbles from app: whose frame it is, and bubbles that say who fizzy#475);
• joining mid-session from a track's checkpoint;
• what is shared and what stays per person: layout, panes, selection, undo.