Native Multi-Value Indexes for JSON

Summary & Strategic Imperative: MariaDB currently lacks the ability to index JSON arrays efficiently, restricting users to strict 1:1 row-to-index mappings. This limitation forces developers to rely on slow full-table scans or cumbersome schema normalization for common tasks like tagging or log analysis, creating a critical competitive gap with MySQL 8.0, Oracle, and postgres. To reclaim these workloads, MariaDB must implement JSON indexing on top of MDEV-25848, enabling for example a single row containing a JSON array (e.g., [1, 2, 3]) to generate multiple index entries. This feature is essential for semi-structured applications that require high-performance filtering on list-based attributes.