Pin Jena to 5.6.0 (arq 6.x breaks quad CONSTRUCT)

arq 6.x cannot output quad CONSTRUCT results ("No dataset writer for
Turtle_pretty", exit 2; --results=trig was dropped), which the graphify
stage depends on — see apache/jena#4091.

- etl/config.mk: default JENA_HOME → apache-jena-5.6.0, with a comment
  explaining the constraint.
- CLAUDE.md: document the Jena 5.6.0 prerequisite and the arq bug.
- docker-compose.override.yml: enable the image pin
  (atomgraph/linkeddatahub:fe7106d…).
- datasets/current/taxonomies/{legal-forms,legal-statuses}.trig:
  ETL regeneration under Jena 5.6.0 — reordering only, no semantic
  change (round-trips identically via riot --output=nquads | sort).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: namedgraph

CLAUDE.md

@@ -40,7 +40,10 @@ make down / make drop          # stop stack / wipe LDH runtime state (never data
 ```
 
 Prerequisites: Docker (only for `atomgraph/csv2rdf`; no docker-compose), Apache Jena
-(`JENA_HOME`, Jena 6 needs Java 21+), `xsltproc`, `uv`, `make`, `curl`. The `make
+**5.6.0** (`JENA_HOME` — arq 6.x cannot output quad CONSTRUCT results at all:
+"No dataset writer for Turtle_pretty", `--results=trig` dropped; see
+[apache/jena#4091](https://github.com/apache/jena/issues/4091)), `xsltproc`,
+`uv`, `make`, `curl`. The `make
 sef` target additionally needs Node/`npx` (`xslt3-he`, the Saxon-JS compiler) and
 `xmlstarlet`.