Replace media builders with mutable collections.

[ioquatix]

Summary

• remove Set/Map builder and block-builder APIs
• support direct incremental Set/Map mutation followed by freeze
• lazily compile and invalidate lookup caches before freezing
• make Map wildcard lookup derive from effective exact registrations
• expose consistent universal-set detection
• update guides for mutable construction

Tests

• bundle exec sus
• bundle exec rubocop
• bundle exec bake covered:validate --paths .covered.db
• bundle exec bake decode:index:coverage lib
• git diff --check