Achieve 100% Ruby test coverage

[samuel-williams-shopify]

Summary

• add focused coverage for renderer flags, helper methods, and header traversal
• add Node#next_header, retain Node#next_heading as a compatibility alias, and recognize :header nodes correctly
• make the base renderer raise NotImplementedError for unimplemented code blocks
• use the stable CGI.escapeHTML API supported by Ruby 3.4 and newer
• remove the vestigial blocksep, containersep, and @need_blocksep renderer machinery
• document the behavior fixes in releases.md

Validation

• bundle exec bake test — 2224 tests, 2362 assertions
• COVERAGE=PartialSummary bundle exec bake test — 403/403 lines, 100.0%
• bundle exec bake covered:validate --paths .covered.db
• bundle exec bake decode:index:coverage lib — 69/69 public definitions documented
• RuboCop passes for all changed Ruby files