data differ tool - validate accurately when source is still taking writes

it is not always possible to run long running validation scripts during maintenance window downtime. Any design of being able to validate data without stopping source traffic would be beneficial.

Start by reading the data differ tool's validation flow and how it handles a source that is still receiving writes. Define what accurate validation during active source traffic should mean, then document and verify the design against long-running validation outside a maintenance window.

Written by the indexing model from the issue text.