feat(content): add Sol city data to sol_markers/ for Luna, Mars, Europa (#849)

Adds city sections to three sol_markers/ feature files so sol_import.py
produces inhabited bodies with atlas_cities coverage.

luna_features.json (GJ0d-1, 350M pop): 3 cities — Artemis (capital, south
polar region, water ice access, 200M), Tranquility Station (Mare
Tranquillitatis, historic first settlement, 90M), Selene (Mare Imbrium,
industrial hub, 60M).

mars_features.json (GJ0e, 1.2B pop): 4 cities — Olympia (capital, Olympus
Mons region, lava tube habitat, 450M), Hellas Station (Hellas Basin, deep
low-pressure warmth, 350M), Elysium City (Elysium Mons, 250M), Chryse
Landing (Chryse Planitia, historic Viking site and port gateway, 150M).

outer_features.json (GJ0f-2 Europa, 30M pop): 2 cities — Conamara Station
(capital, Conamara Chaos, geothermal near ice plate faults, 20M), Pwyll
Base (Pwyll Crater, sub-ice ocean access, 10M).

earth_features.json already had comprehensive city coverage (40+ cities)
and required no changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tooling/planet-gen/sol_markers/luna_features.json

@@ -1,5 +1,10 @@
 {
     "GJ0d-1": {
+        "cities": [
+            {"name": "Artemis",             "center": [215, 250], "population": 200000000, "kind": "capital"},
+            {"name": "Tranquility Station", "center": [119, 282], "population":  90000000, "kind": "city"},
+            {"name": "Selene",              "center": [93,  247], "population":  60000000, "kind": "city"}
+        ],
         "pois": [
             {"id": "poi_mare_tranquillitatis",  "name": "Mare Tranquillitatis",  "center": [119, 282], "kind": "mare"},
             {"id": "poi_mare_imbrium",           "name": "Mare Imbrium",           "center": [93, 247],  "kind": "mare"},

tooling/planet-gen/sol_markers/mars_features.json

@@ -1,5 +1,11 @@
 {
     "GJ0e": {
+        "cities": [
+            {"name": "Olympia",         "center": [107, 358], "population": 450000000, "kind": "capital"},
+            {"name": "Hellas Station",  "center": [148, 329], "population": 350000000, "kind": "city"},
+            {"name": "Elysium City",    "center": [103, 413], "population": 250000000, "kind": "city"},
+            {"name": "Chryse Landing",  "center": [100, 392], "population": 150000000, "kind": "city"}
+        ],
         "mountain_ranges": [
             {"name": "Olympus Mons",     "peak": [107, 358], "center": [107, 358]},
             {"name": "Tharsis Bulge",    "peak": [115, 365], "center": [118, 362]},

tooling/planet-gen/sol_markers/outer_features.json

@@ -9,6 +9,10 @@
         ]
     },
     "GJ0f-2": {
+        "cities": [
+            {"name": "Conamara Station", "center": [118, 328], "population": 20000000, "kind": "capital"},
+            {"name": "Pwyll Base",       "center": [155, 328], "population": 10000000, "kind": "city"}
+        ],
         "pois": [
             {"id": "poi_conamara_chaos", "name": "Conamara Chaos", "center": [118, 328], "kind": "chaos"},
             {"id": "poi_pwyll_crater",   "name": "Pwyll Crater",   "center": [155, 328], "kind": "crater"},