Add DID document validation module and fixtures

validation/README.md

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+# TRAIL Protocol - DID Document Validation
+
+This directory contains a validation module for checking whether a `did:trail` DID Document conforms to the validation rules defined for Issue #6 and the core structures described in the TRAIL DID Method specification.
+
+## Files
+
+- `did-document-validator.js` — validation logic
+- `validate.js` — CLI runner
+- `fixtures/` — valid and invalid example DID Documents
+
+## What the validator checks
+
+The validator currently checks:
+
+- required top-level fields:
+  - `id`
+  - `controller`
+  - `verificationMethod`
+  - `authentication`
+- `id` format for:
+  - `did:trail:self:<multibase>`
+  - `did:trail:org:<slug>-<16hex>`
+  - `did:trail:agent:<slug>-<16hex>`
+- `verificationMethod` entries:
+  - required `id`, `type`, `controller`
+  - valid key material via `publicKeyJwk` or `publicKeyMultibase`
+- `authentication` references:
+  - each entry must point to an existing `verificationMethod.id`
+- `service` entries (if present):
+  - valid `id`, `type`, and `serviceEndpoint`
+- `capabilityDelegation` entries (if present):
+  - each entry must reference an existing verification method or a valid `did:trail` DID / DID URL
+- agent-specific validation:
+  - `trail:parentOrganization` must be present and must be a valid `did:trail:org` DID
+
+## Run the CLI
+
+From the repository root:
+
+```bash
+node validation/validate.js validation/fixtures/valid-did-document.json
+
+```
+## Run the automated tests
+
+From the repository root:
+
+```bash
+node --test validation/test.js
+```
+
+The test runner loads all fixtures in `validation/fixtures/` and verifies that:
+- `valid-did-document.json` passes validation
+- all `invalid-*.json` fixtures fail validation