Transact-SQL: Set-Based SQL Technique for Running-List of Accumulated Values (String/Number), with Break-Level Resets

MS SQL Server: Algorithm and Query for Set-Based Running Accumulators (i.e., Accumulate Row-Level Values across multiple Rows) into a String of Delimited-values, with Break-Level Resets
Have you ever wanted to perform a set-based running-subtotal operation or running-accumulation-of-values (across multiple rows) in SQL-Server — i.e., have row-level accumulators store a running total or running