git-recon battles online: a Durable Object per room (price it first) #1184

Follow-up to #1172. apps/git-recon/src/net/world.ts is Durable Object-shaped, so it can run as one DO per (level, owner/repo@ref) online. Not done in #1172 because it needs:

• a Worker in front that vouches X-User-Id / X-User-Name (like park-hide-seek's edge), plus room codes or quick play;
• the setup message sized for big repos (the host's page sends the snapshot now);
• pricing first: DO duration at 20 Hz while a room is open, plus snapshot egress (see the deltas issue). Keep the first deploy the smallest live resource.