/live sends everyone a coalesced presence frame {type:'presence', here, signedIn, watching} within 1 s of a connect or disconnect - lobby and test only #358

Split from #291, which was parked after its preview deploy failed (pull #295). This is the server half only: the page does not change.

Done:

• [ ] Software/so101/src/lobby.ts keeps the counts (every open /live socket, the signed-in people counted once each, and anonymous watchers), and broadcasts {type:'presence', here, signedIn, watching} at most once per second after a change. A burst of connects sends one frame. A new socket gets the current frame at once.
• [ ] No names and no addresses are in the frame.
• [ ] Tests in src/lobby.test.ts: the counts rise on connect and fall on disconnect, two tabs of one signed-in person count once in signedIn, and a burst of five connects is one frame.
• [ ] Only lobby.ts and lobby.test.ts change. forge.json is untouched, and bun --cwd Software/so101 test passes.