Blurry placeholder performance: cache on disk

It would be amazing if the blurry placeholder LRU could be optionally backed by a disk cache. That would make build-performance much faster where the node process is killed between builds.

Start by locating the blurry placeholder LRU and the build path that creates it; confirm how the Node process is restarted between builds. Read the existing cache behavior first, then determine the option and persistence behavior for a disk cache. Done means repeated builds can reuse placeholders from disk when enabled without changing the default behavior.

Written by the indexing model from the issue text.