Feature Request: Expose Window Open and Close Timestamps

Feature Request: Expose Window Open and Close Timestamps: a task in LegoFlow-SWE (Harbor dataset). Problem Windowing operators (e.g., fold window , reduce window ) currently emit only (key, values) tuples. Users have no access to the window’s start and end timestamps. This makes it impossible to…

The task

**Problem** Windowing operators (e.g., `fold_window`, `reduce_window`) currently emit only `(key, values)` tuples. Users have no access to the window’s start and end timestamps. This makes it impossible to reliably use window boundaries as keys for downstream pivoting or grouping operations.

Part of Lego-X/LegoFlow-SWE.