Busy sandboxes return unused file cache to the server

A sandbox's kernel keeps every file it reads or writes in memory until that memory is needed. Previously this cache was released only after the sandbox had been idle for 5 seconds, so a sandbox that worked continuously, such as a CI job, held its full memory size on the server. The sandbox's agent now releases the coldest clean file cache every 10 seconds while the sandbox works: a quarter of the cache above one sixteenth of its memory each time. It pauses whenever the sandbox reads that cache back, so a workload that rereads the same files keeps them in memory. Unwritten data is never released. More sandboxes fit on each server, and creates and wakes are refused for lack of memory less often. The resident memory that GET /v1/sandboxes/{id}/metrics reports for a busy sandbox drops to what it uses, plus that cache floor. Memory pricing and limits are unchanged.