Migrate Google Script to V8

My current standalone Apps Script powers several custom functions inside a Google Sheets workbook, but it still runs on the legacy runtime. I need the code refactored so it executes smoothly in the modern V8 environment without changing the way users call those sheet functions.

Key points you should know:
• Scope is limited to the in-sheet functions; the script does not touch external APIs or services.
• All existing behaviour, argument signatures and return values must remain intact so spreadsheets already in use stay fully compatible.
• Any deprecated syntax (e.g., var-scoped functions, old-style for-in loops on arrays, etc.) has to be replaced with V8-safe patterns while keeping performance at least equal to the current version.
• After migration, you will share the updated .gs files and a brief changelog highlighting each significant modification, plus a short note on any potential future optimisation.

I’ll test the converted script by running it in a fresh spreadsheet. Acceptance is complete when every legacy function returns identical results, and the Apps Script dashboard confirms it is executing in the V8 runtime with no warnings.