Step 6: Two people at once
The same code, mounted twice — and you didn't write a line of sync.
No new code this step. Instead, watch what the code you already wrote does when two people have it open at once.
Below is the exact tool from Step 5, mounted in two independent panes — two separate copies, exactly as if two people had it open on two different computers.
You should see: add a note in the left pane and it appears on the right — not copied over, arrived. Now add one in both panes at once, as fast as you can. You get two new notes, not one overwritten by the other.
You didn't write any sync code. field.map already merges concurrent writes by key —
Step 3's one-line decision is the entire reason this works. It's also why the codec lives in
its own file: the shape is the part the platform has to merge across people; each pane's
view just reads the merged result.
One honest caveat: two people typing into the same note is per-key last-write-wins — a
keystroke sets the whole note — and field.prose is the per-character field type for real
co-typing. The four field types covers when to reach for it.