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>

decisions/architecture.md

@@ -645,4 +645,13 @@ Technical foundation decisions that constrain implementation: engine, client-ser
 
 ---
 
-*50 decisions. Last updated: 2026-03-24 (D-101 partial supersession noted; D-166 development cascade added)*
+### D-188: Rename biome_summary to planet_class across codebase
+- **Date:** 2026-04-06
+- **Decision:** The `biome_summary` field in `systems.db` and all references throughout the codebase are renamed to `planet_class`. "Biome" describes per-zone vegetation classification (Whittaker table — tropical rainforest, temperate deciduous, etc.). "Planet class" describes the overall planetary character (temperate, arid, frozen, etc.). The two concepts were conflated under one name, causing confusion in the planet generator pipeline where both exist.
+- **Scope:** DB column (`bodies.biome_summary` → `bodies.planet_class`), schema SQL, all Rust atlas code, wiki table headers (`Biome` → `Class`), atlas proposal JSONs, design docs, decision refs, planet generator scripts.
+- **Raised by:** Team Leader (Jeroen), during planet generator spike.
+- **Dissent:** None.
+
+---
+
+*51 decisions. Last updated: 2026-04-06 (D-188 biome_summary → planet_class rename)*