Plugin installs no longer silently drop a node

Installing a plugin writes its nodes in batches, and each batch is classified as "new" from a snapshot taken at the start of the install. If something else wrote one of those paths in the meantime, the storage layer correctly refused the batch's write for that node — but the install counted it as written anyway, so the partition kept the older content while the install reported success. Nothing surfaced it: the node only got its real content on the next install of the same package.

The installer now notices the refusal and re-applies the node the same way it applies any node that already exists — honouring a node you have claimed as your own, skipping one that is already identical, and otherwise landing the package's content. So a plugin's content is what the package says after one install, and re-installing an unchanged package really does nothing.