Follow LinkedDataHub off the LDT and apps namespaces

[namedgraph]

LinkedDataHub 6.0.0 retires https://www.w3.org/ns/ldt and renames its deployment-configuration ontology — AtomGraph/LinkedDataHub#394. Both are breaking for stored data as well as for stylesheets, and this repo carries both: every document in app/ and datasets/ is typed dh:Container or dh:Item, and the dataspace descriptions in config/ are typed and wired with lapp: and ldt: terms.

dh: moves host without changing shape

https://www.w3.org/ns/ldt/document-hierarchy# → https://w3id.org/atomgraph/linkeddatahub/document-hierarchy#

The dh: prefix and the Container, Item and slug local names are untouched, so across the 57 files holding app data, ETL mappings and SHACL shapes only the namespace string differs — the diff is one line per prefix declaration, not one per term.

lapp: becomes lds:

https://w3id.org/atomgraph/linkeddatahub/apps# → https://w3id.org/atomgraph/linkeddatahub/dataspaces#

• config/dataspaces.trig and config/system.trig type their dataspaces lds:Dataspace, lds:EndUserDataspace, lds:AdminDataspace and wire them with lds:origin, lds:ontology, lds:service
• ldt:ontology and ldt:service move into that same vocabulary, so each config file now needs one prefix where it needed two
• files/layout.xsl: lapp:origin() → lds:origin(), and the footer's $ldt:base — a parameter LinkedDataHub retired before this change — → lds:base()

The client SEF path from #26 is untouched; this only moves the function names.

Verification

• files/layout.xsl compiles under Saxon-HE Java and files/client.xsl exports to SEF, both against a LinkedDataHub tree built from the rename. The SSR check has to go through Saxon-HE: SaxonJS cannot compile the server-side stylesheet, because the use-when duals leave ac:langs() undefined for it.
• All 58 Turtle, TriG and SPARQL files in this commit parse with riot and qparse.

Not in this PR

• files/client.xsl.sef.json still needs rebuilding. It is untracked build output, and make sef has to run against a LinkedDataHub image that carries the rename — rebuilding it against the current published image would bake the old namespaces back in.
• Nine files with uncommitted local work are deliberately left out, so this branch is only the rename: CLAUDE.md, Makefile, README.md, app/.ldhignore, app/import-ns.sh, app/install.sh, the app/update-folder.sh deletion, docker-compose.override.yml, and the bin/make-public.sh comment reword. Where the rename touched those files it touched text that is itself uncommitted — CLAUDE.md's $ldt:base gotcha, for instance — so those touch-ups will ride along when that work is committed. bin/make-public.sh is the one exception: its dh: prefix line is committed here, its comment reword is not.
• Merged master (Serve the client SEF from static/lt/linkeddata/ instead of static/com/ltlod/ #26) is included — this branch is a fast-forward from bde764d.

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