Add entities multilang support with lazy re-keying and pure function …

[mdon]

…tests

• Add Multilang module for multi-language JSONB data transformations
• Add language tabs, ghost text placeholders, and field separation in entity/data forms
• Add lazy primary language re-keying on edit when global primary changes
• Add translation convenience API for entities and entity data (programmatic use)
• Fix title translation functions to use embedded primary instead of global
• Fix merge_multilang_data to preserve multilang structure when Languages disabled
• Fix extract_unique_categories and navigator templates for multilang data
• Add 124 pure function tests for Multilang, HtmlSanitizer, FieldTypes, FieldType
• Update documentation (README, OVERVIEW, DEEP_DIVE, Languages README, CHANGELOG)