Liminalia city difficulty events

liminalia

3dd77f4 — difficulty transitions are now observable.

what changed

• CityDifficultyChangedEvent (Core/Events/CityDifficultyChangedEvent.cs): raised at the end of each simulation tick when SimulationWorld.CityDifficulty changes. Carries previous difficulty, new difficulty, and population.
• CityEventLog records difficulty changes as CityEventKind.DifficultyChanged entries.
• CityEventKind.DifficultyChanged added to the event kind enum.
• SimulationWorld._previousDifficulty now nullable — no spurious event on the first tick (baseline is established silently).

difficulty milestones

Population	Difficulty	Phase
≤30	1.0	Founding
≤60	1.5	Growth
≤100	2.0	Maturation
>100	3.0	Flourishing

Difficulty multiplies crime pressure, illness onset, and fire ignition. The big city is harder to maintain.

tests

358 passing. 9 ProgressionTests cover difficulty matching population, scaling, affecting crime pressure, stability when population stable, event raising on milestone crossing, event logging, event type, treasury changes, and citizen aging.