feat(content): add typed item relationships

[gcomneno]

Summary

• add an optional, ordered and domain-neutral relations contract to catalog item YAML;
• normalize and expose typed relationships through the standard item content layer;
• validate malformed relationship structures while preserving legacy items without relations;
• preserve existing relationships when an item is edited through Studio;
• document the contract with non-genealogical examples.

Relationship contract

relations:
  - type: inspired-by
    target: earlier-work
    label: Earlier study
  - type: part-of
    target: summer-project

• type and target are required non-empty strings;
• label is optional;
• values are trimmed and blank labels are omitted;
• relation order is preserved;
• missing relations is normalized to [].

Validation in this issue is intentionally structural only. Target existence, self-references, duplicates, inverse relationships and graph semantics remain outside this scope.

Verification

• focused relationship tests: 2 passed, 0 failed;
• full test suite: 301 passed, 0 failed;
• npm run content:validate;
• npm run check: 0 errors and 0 warnings;
• npm run build;
• git diff --check.

Closes #206