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>

server/data/systems-schema.sql

@@ -144,7 +144,7 @@ CREATE TABLE IF NOT EXISTS bodies (
     surface_gravity REAL,              -- in g (Earth = 1.0), NULL for gas giants
     orbital_period_days REAL,          -- orbital period in Earth days, NULL for oort/belt
     rotation_period_hours REAL,        -- rotation period in Earth hours (Earth = 24), NULL for tidally locked/gas giants
-    biome_summary TEXT,                -- temperate, arid, frozen, oceanic, volcanic, barren, mixed
+    planet_class TEXT,                 -- temperate, arid, frozen, oceanic, volcanic, barren, etc.
     hydrosphere TEXT,                  -- ocean, ice, rivers, none, subsurface
 
     -- Settlement