fix(db): address PR #144 review (#1000)

- Update D-216 amendment text to match the implemented decision:
  events-first uses event_type='colonial_charter' (9 systems, 11 bodies,
  5 diverging from wave) — not the stale founding/first_settlement/
  colonization scaffolding vocabulary.
- Add ORDER BY sort_order ASC to the colonial_charter subquery for a
  deterministic pick if a system ever has two charter rows.
- Add explicit ELSE NULL to the wave-fallback CASE so an unexpected
  settlement_wave leaves a trace instead of silently nulling.

Data unchanged: 269/273 set, 4 Sol NULL; stamp fresh.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tooling/economy-db/import_economics.py

@@ -380,6 +380,7 @@ SET founding_age_years = COALESCE(
         FROM historical_events he
         WHERE he.system_id = bodies.system_id
           AND he.event_type = 'colonial_charter'
+        ORDER BY he.sort_order ASC
         LIMIT 1
     ),
     (
@@ -389,6 +390,7 @@ SET founding_age_years = COALESCE(
             WHEN 'wave_3' THEN 300
             WHEN 'wave_4' THEN 100
             WHEN 'wave_5' THEN 40
+            ELSE NULL -- unexpected settlement_wave: add a WHEN above
         END
         FROM system_history sh
         WHERE sh.system_id = bodies.system_id