Refactor participant tracking logic into a dedicated, testable class:
- Create GeohashParticipantTracker with GeohashParticipantContext protocol
- Move GeoPerson struct to new file
- Extract participant recording, refresh, and count logic
- Add 18 comprehensive tests for the tracker
- Update ChatViewModel to use tracker via dependency injection
- Subscribe to tracker's objectWillChange for SwiftUI observation
This reduces coupling in ChatViewModel and makes participant tracking
testable in isolation.
