feat(age): ✨ Introduce new game mechanics and content for the "Age of 4" simulation

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

games/age-of-4/docs/races/HOMEWORLDS.md

@@ -0,0 +1,97 @@
+# Age of Four — Race Homeworlds
+
+> Data lives in `resources/worlds/{homeworld_id}.json` and `resources/races/{race_id}.json`. This doc covers design intent only.
+
+---
+
+## Homeworld System
+
+Every race evolved on a specific planet. Their homeworld determines:
+
+1. **Start bias** — where on the map they prefer to settle (weighted terrain placement)
+2. **Terrain affinity** — per-tile yield/defense/movement bonuses on their preferred terrain, applied globally regardless of map type (defined on the race, not the homeworld)
+3. **Planetary bonuses** — whole-race stat bonuses that activate **only when the active map's world archetype matches their homeworld archetype** (defined on the homeworld)
+
+### Planet Archetypes
+
+Three archetypes are defined in `engine/src/worlds/`:
+
+| Archetype | Gravity | Physics | Fantasy interpretation |
+|-----------|---------|---------|----------------------|
+| `earth` | 1.0g | Full ecology, water cycle, weather | Standard fantasy world |
+| `mars` | 0.38g | Thin atmosphere, dust storms, no ecology | Harsh, arid, low-pressure survival world |
+| `venus` | 0.91g | Dense atmosphere, volcanism, acid clouds | High-pressure, magically saturated world |
+
+**Gravity is mechanical.** Races evolved for low gravity (mars) move faster and have longer ranged attack on mars-type maps. Dense-atmosphere races (venus) gain magic range and power on venus-type maps.
+
+---
+
+## The Four Races
+
+### High Elves — Silvandel (`venus`)
+
+> *Before the runaway greenhouse took hold, Silvandel was a world of impossible beauty — dense with mana, draped in luminous cloud forests, every breath charged with arcane potential.*
+
+- **Planet:** Venus-type — dense, pressurized, high mana density
+- **Dominant biomes:** `enchanted_forest`, `alpine_meadow`, `coral_reef`
+- **Climate:** Warm, wet, high-pressure
+- **Start bias:** Forests, coastal highlands (weight 3)
+- **Planetary bonus (venus maps):** Magic +2, magic_range +2 — dense atmosphere resonates with elvish spellcraft
+- **Terrain affinity (global):** `temperate_forest` → culture +2, magic +1 · `enchanted_forest` → magic +3
+
+High Elves breathe dense magical atmosphere natively. On venus-type maps they are at their most powerful — spells carry further and resonate harder. On earth-type maps they rely on terrain affinity in forests alone.
+
+---
+
+### Humans — Valorheim (`earth`)
+
+> *Valorheim was a balanced world — no extreme, no single advantage. It forged a people who could adapt to anything.*
+
+- **Planet:** Earth-type — standard gravity, full ecology, mixed biomes
+- **Dominant biomes:** `temperate_grassland`, `temperate_forest`, `plains`
+- **Climate:** Temperate, balanced moisture
+- **Start bias:** Temperate mixed terrain (weight 2, low preference — humans adapt anywhere)
+- **Planetary bonus (earth maps):** Production +1, food +1 — mild across-the-board competence
+- **Terrain affinity (global):** `temperate_grassland` → food +1 · `plains` → production +1
+
+Humans are the blank-canvas race. Their planetary bonus is intentionally modest — they're supposed to be adaptable, not dominant in any one context.
+
+---
+
+### Dwarves — Khazad Prime (`earth`)
+
+> *Khazad Prime is a cold, mineral-dense world where the mountains are the world — not scenery but home, resource, and fortress combined.*
+
+- **Planet:** Earth-type — standard gravity, but cold and geologically active
+- **Dominant biomes:** `mountains`, `alpine_tundra`, `subterranean`
+- **Climate:** Cold, low moisture, high elevation
+- **Start bias:** Hills and mountains (weight 3)
+- **Planetary bonus (earth maps):** Production +1, fortification_rate +10 — stone-sense and engineering mastery
+- **Terrain affinity (global):** `mountains` → production +2, defense +15 · `hills` → production +1, movement_cost_reduction 1
+
+Dwarves gain their production and defense advantages on any map that has mountains. On earth-type maps, their planetary bonus stacks, making mountain fortresses nearly impregnable.
+
+---
+
+### Orcs — Gorgrund (`earth`)
+
+> *Gorgrund is a world in geological adolescence — volcanic, unstable, punishing. The orcs didn't conquer it. They became it.*
+
+- **Planet:** Earth-type (volcanically active variant) — lava fields, rocky badlands, harsh climate
+- **Dominant biomes:** `volcano`, `rocky`, `savanna`
+- **Climate:** Hot, arid to semi-arid, seismically active
+- **Start bias:** Open terrain, volcanic highlands (weight 3)
+- **Planetary bonus (earth maps):** Growth +2, expansion_cost -10 — orcs spread like fire on familiar ground
+- **Terrain affinity (global):** `lava_fields` → growth +2 · `rocky` → expansion +1
+
+> **Design note:** Gorgrund's world_archetype is `earth` (volcanically active), not `venus` (acid/pressure). The volcanic activity on Gorgrund is geological, not atmospheric. This may be revisited — an argument exists for a fourth archetype (volcanic world) if orcs and draconians need different planetary bonuses.
+
+---
+
+## Bonus Stacking
+
+Terrain affinity and planetary bonuses stack:
+- A Dwarf city on mountains, on an earth-type map: +2 prod (affinity) + 1 prod (planetary) + 15 defense (affinity) + 10% fortification rate (planetary)
+- A High Elf city in enchanted_forest, on a venus-type map: +3 magic (affinity) + 2 magic (planetary) + 2 magic_range (planetary)
+
+This creates genuine incentive to play your race on their homeworld type when the map option exists.

games/age-of-4/game.json

@@ -0,0 +1,17 @@
+{
+  "id": "age-of-4",
+  "name": "Age of Four",
+  "version": "1.0.0",
+  "engine_version": ">=1.0.0",
+  "description": "The four founding races — High Elves, Humans, Dwarves, and Orcs — with full 4X gameplay and all 5 magic schools.",
+  "author": "Core Team",
+  "discipline_count": 5,
+  "foundation_count": 6,
+  "era_count": 10,
+  "transit_layers": 0,
+  "playable_races": ["high_elves", "humans", "dwarves", "orcs"],
+  "settings": {
+    "gender_system": true,
+    "society_presets": ["matriarchal", "patriarchal", "random"]
+  }
+}