read_body_settlements propagated a parse error for any non-NULL but
unrecognized settlement_class, which the atlas proxy swallowed and then
enqueued the body with zero cities — one malformed row silently dropped
placement for the entire body. Treat an unknown variant like NULL: fall
back to PopulationBudget with a warning. parse_settlement_class stays
strict for the D-199 read_set path, which must abort on bad fields.
Found in PR #149 review (Hoshe H1). Adds a test covering the fallback.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.8
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