registry: document and validate hybrid registry discovery posture

[mdheller]

Summary

Implements the #93 registry-posture decision as an explicit hybrid discovery model instead of leaving supplemental registries as ambiguous transitional artifacts.

Adds

• docs/registry-discovery-posture.md

  • states that catalog/registry.ttl remains the primary registry
  • states that supplemental registry files under catalog/ are first-class discovery inputs for now
  • defines consumer discovery rule for registry.ttl, *registry*.ttl, and *.registry.ttl
  • states release guidance: consumers must read primary plus supplemental registries until normalization is complete

• scripts/validate_registry_discovery.py

  • discovers registry-like Turtle files under catalog/
  • parses them
  • finds og:Module records
  • fails if no module records are found
  • fails if a discovered module record lacks og:path

• Makefile

  • adds validate-registry-discovery
  • wires it into make validate

Registry posture

This PR does not force a central-only registry migration. It codifies the current state as:

hybrid-discovery-now, normalization-decision-later

A future normalization PR can still fold supplemental registry entries into catalog/registry.ttl or permanently adopt hybrid discovery.

Validation policy

Let CI run. If the new validator exposes malformed registry records, remediate them instead of weakening discovery semantics.

Related

Closes #93 after merge if validation passes.