refactor(schema): rename biome_summary to planet_class (D-188)

"Biome" describes per-zone vegetation classification (Whittaker table).
"Planet class" describes overall planetary character. The conflation
caused the planet generator to misclassify ~270 bodies as barren.

Scope: systems.db column, schema SQL, Rust atlas code, wiki table
headers (Biome → Class), atlas proposal JSONs, all docs/decisions,
tooling scripts. Also normalizes atmosphere vocabulary (breathable →
standard) and expands planet class mapping to all 26 wiki values.
Unknown classes default to temperate for modder safety.

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

docs/atlas/proposals/GJ656.json

@@ -25,7 +25,7 @@
       "orbital_period_days": 10.8,
       "rotation_period_hours": 259.2,
       "atmosphere": "none",
-      "biome_summary": "barren",
+      "planet_class": "barren",
       "hydrosphere": null,
       "economic_role": null,
       "settlement_pattern": null,
@@ -45,7 +45,7 @@
       "orbital_period_days": 68.0,
       "rotation_period_hours": 26.8,
       "atmosphere": "standard",
-      "biome_summary": "cold_arid",
+      "planet_class": "cold_arid",
       "hydrosphere": "liquid_water",
       "economic_role": null,
       "settlement_pattern": null,
@@ -65,7 +65,7 @@
       "orbital_period_days": null,
       "rotation_period_hours": null,
       "atmosphere": null,
-      "biome_summary": null,
+      "planet_class": null,
       "hydrosphere": null,
       "economic_role": null,
       "settlement_pattern": null,
@@ -85,7 +85,7 @@
       "orbital_period_days": 230.0,
       "rotation_period_hours": 25.4,
       "atmosphere": "thin",
-      "biome_summary": "frozen",
+      "planet_class": "frozen",
       "hydrosphere": null,
       "economic_role": null,
       "settlement_pattern": null,
@@ -105,7 +105,7 @@
       "orbital_period_days": 700.0,
       "rotation_period_hours": 29.8,
       "atmosphere": "trace",
-      "biome_summary": "frozen",
+      "planet_class": "frozen",
       "hydrosphere": "ice",
       "economic_role": null,
       "settlement_pattern": null,
@@ -125,7 +125,7 @@
       "orbital_period_days": 1900.0,
       "rotation_period_hours": 22.6,
       "atmosphere": "none",
-      "biome_summary": "frozen",
+      "planet_class": "frozen",
       "hydrosphere": "ice",
       "economic_role": null,
       "settlement_pattern": null,
@@ -145,7 +145,7 @@
       "orbital_period_days": 5000.0,
       "rotation_period_hours": 18.4,
       "atmosphere": "none",
-      "biome_summary": "frozen",
+      "planet_class": "frozen",
       "hydrosphere": "ice",
       "economic_role": null,
       "settlement_pattern": null,
@@ -165,7 +165,7 @@
       "orbital_period_days": null,
       "rotation_period_hours": null,
       "atmosphere": null,
-      "biome_summary": null,
+      "planet_class": null,
       "hydrosphere": null,
       "economic_role": null,
       "settlement_pattern": null,