Disambiguation of attribute semantic names

The [(Meshes) Overview](https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#meshes-overview) section of the specification says

> Application-specific attribute semantics **MUST** start with an underscore, e.g., `_TEMPERATURE`

Some questions are not sufficiently answered by this.

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The first question is very vague and hard to answer specifically: What constitutes "application-specific"?

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A broader, more specific, and somewhat crucial question is: Is it possible to disambiguate custom attribute names in extensions?

For example, there could be two extensions that both define a generic attribute name like `_ID` or `_VALUE`. The name clash would prevent the extensions to be used at the same time. A seemingly simple and straightforward solution would be to require the attribute names to be prefixed with the extension name. Something like `"_EXT_foo_bar_ID"` looks a bit odd, but is as unambiguous as it gets. In any case: _Only_ requiring the undescore prefix may not be sufficient (also because there is no way to sensibly figure out which extension might already use which attribute names).

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A question that is somewhat tangential, but brought up this one (via https://github.com/CesiumGS/3d-tiles/issues/611 ): Can extensions define custom attributes _without_ the `_` underscore?

Based on my understanding, the answer to this is **no**: The validator dedicatedly checks for the semantics that are defined in the core spec, and only skips the ones that start with an undescore, via the [`checkAttributeSemanticName`](https://github.com/KhronosGroup/glTF-Validator/blob/e60dd6d6a96aed2fa18e80af2bfffe5436684438/lib/src/base/mesh.dart#L171) function, causing a `MESH_PRIMITIVE_INVALID_ATTRIBUTE` error for **all** unknown attribute names that do **not** start with an underscore. But I wanted to ask for a confirmation here: Is it correct that such attribute names would be considered as invalid _even when_ the support for the extension was added to the validator?

Start with the Meshes Overview section of the glTF 2.0 specification and the validator entry point checkAttributeSemanticName in lib/src/base/mesh.dart. Review the linked 3D Tiles discussion and comment thread for context. Done means the specification and validator behavior have an agreed, unambiguous resolution for extension-defined attribute names.

Written by the indexing model from the issue text.