Add DID document validation module and fixtures

[AmeyParle]

Summary

This PR adds a validation module for checking whether did:trail DID Documents conform to the validation requirements described in Issue #6.

Changes

• added validation/did-document-validator.js with validation for:

  • required top-level fields (id, controller, verificationMethod, authentication)
  • did:trail identifier format
  • verificationMethod entries and key material
  • authentication references
  • service entries
  • capabilityDelegation structure
  • agent-specific trail:parentOrganization

• added validation/validate.js as a CLI runner

• added validation/fixtures/ with one valid fixture and multiple invalid fixtures

• added validation/README.md with usage instructions and references to the relevant spec sections

Validation

Tested the CLI against:

• valid-did-document.json
• invalid-missing-id.json
• invalid-missing-controller.json
• invalid-bad-id-format.json
• invalid-missing-verification-method.json
• invalid-bad-key-material.json
• invalid-bad-service-endpoint.json
• invalid-bad-capability-delegation.json
• invalid-agent-missing-parent-org.json

Notes

• This validator follows the issue requirement to treat top-level controller as required.
• The valid fixture uses an extended agent-style DID Document so the required-field checks and agent-specific validation can be exercised together.

Refs #6

[chrishooooo-netizen]

Great work @AmeyParle — this is a solid foundation for DID Document validation. I ran all fixtures locally and they pass cleanly. A few notes:

What works well:

• Clean separation between validation functions (composable, testable)
• Good fixture coverage: 8 invalid cases + 1 valid, covering required fields, ID format, key material, services, capability delegation, and agent-specific rules
• The trail:parentOrganization check for agent DIDs is a nice catch
• CLI runner is straightforward and useful

Requested changes:

1. Add automated tests. The validator is core infrastructure — we need a test runner (e.g. a simple Node.js script using assert, or a lightweight framework like node:test) that programmatically runs all fixtures and checks expected outcomes. Right now validation correctness depends on someone manually running each fixture and reading the output. A test.js or npm test setup that exits non-zero on failure would make this much more robust.

2. Add trailing newlines. did-document-validator.js, README.md, and all fixture JSON files are missing a final newline. Small thing, but keeps diffs clean and follows POSIX convention.

Optional improvements (not blocking):

• Consider a fixture for a valid did:trail:self:z... document (currently only agent is tested as valid)
• Consider a fixture for a valid did:trail:org:... document
• The serviceEndpoint validation could eventually support arrays and maps per DID Core, but string-only is fine for now

Contributor scope note:
We're thrilled to have community contributions in examples/, validation/, tests/, and docs/. These areas are open and we actively welcome PRs. Normative spec changes and security-critical components (trust model, capability credential schema, key management) go through maintainer review only — just so expectations are clear as the project grows.

Looking forward to the test additions — once those are in, this is ready to merge. Keep up the great work!

[AmeyParle]

Thanks for the review — I’ve added an automated test runner for the fixtures and cleaned up the trailing newlines. The validator tests now pass with node --test validation/test.js.

I’ve held off on the optional extra valid fixtures for now to keep this PR focused, but I’d be happy to add them in a follow-up if useful.