Pass-through SaveImage variant with accumulating previews and a
promote/lock feature. The node:
- Saves images and passes the input tensor through as the output, so it
fits naturally mid-graph (unlike core SaveImage which is a sink).
- Exposes an 'accumulate' flag, mirroring upstream PR #12647 — the
frontend uses this to append previews to a per-node gallery instead
of replacing it.
- Accepts an optional 'promoted_asset_ref' STRING widget that the
frontend writes when the user clicks a 'lock' UI on a preview. When
set, the node skips saving, loads the referenced image from
output/input/temp, and outputs that image. Stale refs silently fall
back to pass-through.
- IS_CHANGED returns a ref-derived key (incl. file mtime) when locked,
so re-queues with the same lock are cache hits and upstream ancestors
are skipped. Unlocked, it defers to normal input-signature caching.
Includes unit tests covering ref parsing (incl. path-traversal and
symlink-escape rejection), path resolution, pass-through and locked
execution, and IS_CHANGED behavior. 24/24 pass; ruff clean.
