Inconsistent definitions of feature ID

In some places, parry uses the `FeatureId` enum, but `TrackedContent` uses a plain `u32` for its feature IDs. This is also easily confused with the subshape `u32`s in `ContactManifold`, which identify logical shapes but not individual geometric features of a single shape (i.e. not necessarily a specific face, edge, or vertex). It's also unclear whether feature IDs in contact manifolds are scoped to shapes or subshapes, whereas `FeatureId`s necessarily must be shape-scoped since they're e.g. returned by `PointQuery`.

See also [Discord discussion](https://discord.com/channels/507548572338880513/804693334995238962/811171943961198612). Opening an issue for tracking purposes.