feat: move prepopulation guards into participant service

docs/service-simplification-plan.md

@@ -19,6 +19,8 @@ The codebase is already in a better place than it was at the start of the refact
 
 - `BookingService` no longer owns session lifecycle, baseline snapshot handling, return URL management, or room grouping. That work now lives in `BookingSessionService` and `BookingRoomSelectionService`, which keeps the booking orchestration boundary narrower.
 - `BookingService` still covers hydration, booking bootstrap, service preselection, and booking status rules.
+- `BookingParticipantCountService` now handles only participant count shaping.
+- `ParticipantPrepopulationService` now owns applicant prefill plus the create-mode dummy-data shortcut.
 - `BookingPriceCalculatorService` is focused on pricing, but it still sits close to display-oriented behavior in adjacent code paths.
 - `TravelDataService` remains broad and is likely the next larger boundary after booking orchestration is reduced.
 
@@ -104,7 +106,7 @@ Likely directions, only if justified later:
 |------|--------|-------|
 | Participant card DTO cleanup | Done | Card data now uses typed DTOs instead of nested array payloads |
 | Room label formatting cleanup | Done | Pricing labels now have a dedicated presentation helper |
-| Booking service split | In progress | Session lifecycle, baseline snapshot, return URL handling, room grouping, and participant count shaping moved out of `BookingService` |
+| Booking service split | In progress | Session lifecycle, baseline snapshot, return URL handling, room grouping, and participant count shaping moved out of `BookingService`; dummy prefill moved into `ParticipantPrepopulationService` |
 | Pricing service review | Pending | Keep focused on calculation, not rendering |
 | Travel data service review | Pending | Broad boundary, likely later pass |
 | Participant field registry review | Deferred | Real orchestration boundary, intentionally left alone for now |