Add FlatExpression POC: data-oriented flat expression tree with tests

Agent-Logs-Url: https://github.com/dadhi/FastExpressionCompiler/sessions/ac75b2c5-61a5-4e7b-b468-5bfff97971ac

Co-authored-by: dadhi <39516+dadhi@users.noreply.github.com>

Author: Copilot