test(rsc): add persistent use cache example

[hi-ogawa]

• closes Add use cache example with external persistence beyond in-memory store #1409
• closes Research "use cache" cache invalidation strategy during dev #1395

The existing use cache examples keep entries in function-keyed memory, which doesn't exercise realistic cache implementation and does not expose persistent-cache specific requirements for plugin side.

This PR adds a standalone file-level use cache example with a filesystem-backed handler. This demonstrates dev cache invalidation via transform time tagging. Additionally hotUpdate propagates invalidation through module graph for use cache module.