Register the habitat export's column contract in crate (nge/habitat/fwa/v1/segment) #333

If done: the habitat export's column contract is registered in crate as nge/habitat/fwa/v1/segment, the manifest cites crate's tagged YAML as wet's does, and the export conforms through crate::crt_schema_conform(). If never: the contract stays in link (inst/extdata/schemas/habitat_fwa_v1_segment.yaml), checked by link's own .lnk_export_conform().

#308 kept the contract in link, in crate's shape (file, description, canonical: {shape, cols: [{name, type, required, notes}]}), so the move is a copy:

1. Copy the YAML to crate inst/extdata/schemas/nge/habitat/fwa/v1/segment.yaml, add the registry row (schema_only), tag a release.
2. In link: pin crate, read the spec from crate, and keep .lnk_export_conform()'s type check (crt_schema_validate() checks presence only), or move the type check into crate.
3. Manifest datasets[].schema cites https://github.com/NewGraphEnvironment/crate/blob/v<ver>/inst/extdata/schemas/nge/habitat/fwa/v1/segment.yaml.

Do it once the columns stop moving (the has_barriers_dnstr addition in #308 is the latest change).

Relates to #308, NewGraphEnvironment/wet#7.