Implement Block Cache for Filesystem Performance

The hash-table-plus-LRU-list design here is exactly how Linux's unified page cache accelerates every file read, and the clock algorithm mentioned is what the kernel actually uses instead of strict LRU to keep overhead low at scale. Mixing up write-through and write-back semantics is a real source of data-loss bugs: one is safe but slow, the other is fast but loses dirty data on a crash.