Add 'accumulate' BOOLEAN optional input (advanced, default False) to
SaveImage and PreviewImage INPUT_TYPES. Accept the parameter in
save_images() so the execution engine can pass it through.
Set merge flag in the 'executed' websocket message when accumulate is
enabled, so the frontend appends outputs instead of replacing them.
Add unit tests for the accumulate input definition and merge flag
derivation logic.
Amp-Thread-ID: https://ampcode.com/threads/T-019cbb66-bb43-771e-b47b-0f05a436b9cb
