Introduce barbarians as a non-diplomatic civ appended to state.civs,
providing escalating pressure that scales with difficulty and target
civ era. Key mechanics:
- Uprisings spawn on explored tiles, capped by a growing population
limit to keep raids an event rather than a fourth empire
- Era-based pressure curve: raids decay as a civ advances, stopping
entirely at hard-stop techs (industrialization/conscription/railroad)
- Killing hordes summons a Barbarian Leader with an escort; cornering
the lone leader triggers a gold ransom scaled by era and difficulty
- Captured cities are either razed (small) or held inert (large)
- Leader expires after a set lifetime if not ransomed
Diplomacy and AI carefully exclude barbarians to prevent three traps:
shared-enemy attitude pollution, permanent war phase pinning, and
government ladder lockout. Pre-feature saves remain compatible.
New assets: optional civilization-barbarians.png sheet (8 frames).
Updated sprites.md and unit tooltips. Comprehensive verification tests
added in verifyCivilization.js.
