feat(2024): add Classes collection

12 SRD 5.2.1 class documents with Zod schema and tests.
Schema follows the 2014 structure adapted for 2024 URL conventions.
Replaces nyi field in Subclasses with proper class URLs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: t11z