There's no `IsFeatureVisible(featureId)`

A quick search through the codebase will reveal many places such functionality is defined, as part or helper function.

There's no easy way to check if a featureId is visible, except getting its position and checking visibility or checking if the id is included in `GetVisibleFeatures`.

See `GetFeatureDefId` (visibility checks) and `IsUnitVisible` as a reference.

Start by searching the codebase for GetFeatureDefId, IsUnitVisible, and GetVisibleFeatures, comparing how each performs visibility checks. Add the missing IsFeatureVisible(featureId) entry point so callers can check visibility directly, then verify it covers the same visibility behavior as the referenced helpers.

Written by the indexing model from the issue text.