Add DSLO v0.6 registry layer (namespace, prefixes, manifest, PURL config)

Adds the complete DSLO v0.6 registry layer, including manifest.jsonld, prefixes.json, purl-config.yaml, and registry.yaml. These files establish the identifier and namespace infrastructure for DSLO v0.6, providing stable prefixes, artifact mapping, and PURL configuration. This commit completes the registry placeholder surface required for cross‑platform alignment and future import population.

registry.README.md

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+DSLO Registry (v0.6)
+Placeholder — IDSPACE, PURL, and prefix allocation system.