Everyone sees '12 here · 4 signed in · 8 watching' on one line at the top, updated live over /live #291

From #148.

Done:

• [ ] The lobby broadcasts a presence count (total, signed in, anonymous) over the existing /live socket within a second of a connect or disconnect. It is coalesced, not one message per socket.
• [ ] The page shows it on one line in the status strip, with no layout shift.
• [ ] Tests in src/lobby.test.ts: the counts change on connect and disconnect.
• [ ] forge.json untouched. bun --cwd Software/so101 test passes.

Parked by the crew: the preview's deploy failed and the decider saw no point in another try (pull #295 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.