Validator: cross-check conditions.feature keys against featureConditions catalogue

Surfaced in PR #29 review. `parseCharacter` validates `conditions.feature` (a `Record`) and `featureConditions` (a `string[]` catalogue) independently. A hand-crafted JSON can have keys in `conditions.feature` that aren't declared in `featureConditions` — the import succeeds, but the eventual feature-condition toggle UI will treat the catalogue as truth and silently drop the orphaned booleans.

Once the feature-conditions UI lands, the validator should reject any `conditions.feature` key not present in `featureConditions`. Deferred from #5 because no UI consumes the relationship yet.

Acceptance: parseCharacter rejects with a clear error when `Object.keys(conditions.feature)` contains an id not in `featureConditions`.