Add a fuzzer that creates an on-disk CAS database, stores objects, then
corrupts the on-disk data files using fuzzer-provided bytes and calls
validate(). The goal is that validate() should either succeed or return
an error, never crash.
The fuzzer supports 6 corruption modes: byte-level mutations, file
truncation, appending garbage, zeroing ranges, standalone file
corruption, and combined mutations with continued CAS operations.
Assisted-By: Claude
40d3949162