Validate local and external node references

[sergej-koscejev]

Parent

Parent: #1

What to build

Add structural validation for local and external node references. Local references should resolve against the whole validation target. External references should validate import-index existence and target node ID syntax without checking whether the external target node exists.

Acceptance criteria

• ref@node targets must use supported node ID syntax.
• ref@node targets must exist in the model-wide node ID index for complete validation targets.
• Local ref@to=\":<id>\" targets are checked against the model-wide node ID index for complete validation targets.
• ref@to values without a : separator are reported as structurally invalid.
• External ref@to=\"<import>:<id>\" targets require an existing import index.
• External ref@to target IDs must use supported node ID syntax.
• External target node existence is not checked.
• The dynamic reference marker ^ is rejected in reference targets.
• Tests cover valid local references across root files, missing local targets, missing import indices, malformed target IDs, and external references whose target model is not available.

Blocked by

• Blocked by: Support file-per-root model folders and incomplete root validation #4
• Blocked by: Validate registry and import index structure #5