Implements the three pure functions in registry-schema.cjs (isValidGsdRange,
validateEntries, renderMarkdown) that were stubbed in the previous commit, turning
the red suite green: strict per-type schema validation (capability + eos), a
self-contained engines.gsd range validator (no semver dep), and deterministic
Markdown generation with shields.io release badges + per-entry Discussion links.
Regenerates docs/registries/capability-registry.md and adds the Added changeset.
Also hardens the isValidGsdRange fast-check property (letters-only major) so it
cannot intermittently generate a valid prerelease range and flake.
Closes#2182
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
