Civilization Cycle and Population rework

This update is mainly focused on clearing some tech debt and reworking the Population tab UI. Instead of the previous text dump it should be much clearer and easier to read now.

Not flashy but necessary and beneficial in the long run :)

• Reworked civilization population simulation and UI around a shared Civilization Cycle, with clearer growth, migration, social mobility, needs, historical trends, and cycle-based demographic reporting.

• Added a visible Civilization Cycle progress bar and countdown, showing when the next demographic cycle will be resolved.

• Added Cycle Projections, showing projected Natural Growth, Net Migration, total population change and projected population for the next Civilization Cycle.

• Expanded the Population overview with current population, recent population trend, average change per Civilization Cycle, number of planets and population capacity.

• Preserved and improved both population charts: Social Structure still shows the Poor, Well-off and Rich population split, while Population Trend tracks historical civilization population.

• Migration is now sharded per planet across the Civilization Cycle, spreading calculations over time instead of processing every planet simultaneously.

• Fixed migration limits so a planet can lose at most 5% and receive at most 5% of its population during a Civilization Cycle, with incoming and outgoing migration tracked independently.

• Population-cycle bookkeeping now tracks migration across civilizations directly, allowing the UI to report an actual net migration value per civilization.

• Population projections, planet population information and relevant demographic descriptions are now consistently expressed per Civilization Cycle instead of mixing different tick intervals.

• Updated famine-related population information and other demographic descriptions to use the same Civilization Cycle reference, making population numbers easier to compare throughout the UI.