feat(getters): add GetFloat64 and GetInt64, fix docs

- Add GetFloat64 and GetInt64 methods to ConfigManager and package-level API
- Fix README precedence: actual order is Set > env > file > default
  (was incorrectly documented as file > env > default)
- Fix GetStringMap return type in API table: map[string]any, not map[string]string
- Bump Go to 1.26.1
- Add tests for all new getters (coverage 94.7% → 95.2%)

default.go

@@ -20,6 +20,14 @@ func SetConfigName(name string) {
 	defaultConfigManager.SetConfigName(name)
 }
 
+func GetFloat64(key string) float64 {
+	return defaultConfigManager.GetFloat64(key)
+}
+
+func GetInt64(key string) int64 {
+	return defaultConfigManager.GetInt64(key)
+}
+
 func GetInt(key string) int {
 	return defaultConfigManager.GetInt(key)
 }