[WIP] Implement LET clause similar to OrientDB

[Copilot]

• Inspect contribution guidance, test conventions, and the SELECT execution pipeline
• Add a focused regression test for issue Implement LET clause (like in OrientDB) #210 covering LET before WHERE/SELECT
• Implement minimal parser and execution support for SELECT ... LET ...
• Run targeted tests and format touched files
• Run review, security checks, and secret scanning before finalizing

• Fixes Implement LET clause (like in OrientDB) #210