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/src/bin/atlas/mutate.rs

@@ -22,7 +22,7 @@ pub fn cmd_add_body(
     conn.execute(
         "INSERT INTO bodies (body_id, system_id, parent_body_id, body_type, orbit_index,
                              proper_name, mass_class, atmosphere, surface_gravity,
-                             biome_summary, inhabited, population)
+                             planet_class, inhabited, population)
          VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10, ?11, ?12)",
         params![
             id,