Function default_rule_severities 

pub fn default_rule_severities() -> Value

The zero-config rules.* severities as a flat JSON object keyed by config key (unused-exports, security-sink, …), serialized once from RulesConfig::default(). This is the single source of default severities for fallow schema and the MCP issue-type resource. Infallible in practice (a flat struct of Severity enums); the empty-object fallback keeps callers panic-free and simply yields no default severity if serialization ever changed shape.