Configurable population size & archetype mix ratios

## Context
Population size and archetype distribution aren't user-controllable. `oasis_profile_generator.py` builds the population from `archetype_library.py`.

## Proposed work
- Expose total population size and per-archetype mix ratios as campaign settings.

## Acceptance criteria
- [ ] User can set population size and archetype proportions
- [ ] Generated population respects the configured mix