Validate registry and import index structure

[sergej-koscejev]

Parent

Parent: #1

What to build

Add structural validation for registry and import indices. The validation run should detect duplicate registry/import indices and verify that compact concept and role references resolve against the validation target's own registry.

Acceptance criteria

• Duplicate import indices produce error-severity validation findings.
• Duplicate concept, child role, property role, and reference role registry indices produce error-severity validation findings.
• node@concept values must resolve to concept registry indices.
• node@role values, when present, must resolve to child role registry indices.
• property@role values must resolve to property role registry indices.
• ref@role values must resolve to reference role registry indices.
• Validation does not attempt language-level concept, role ownership, or cardinality checks.
• Tests cover duplicate indices, missing registry indices, and valid registry-backed references through standalone and file-per-root targets where useful.

Blocked by

• Blocked by: Support file-per-root model folders and incomplete root validation #4