copy the caller buffer before in-place moves in zip_files_move

[jmestwa-coder]

In-memory delete mode shifts surviving entry data straight through the caller-owned stream buffer:

• zip_stream_open(..., 'd') aliases the caller's buffer as m_pMem with m_mem_capacity 0
• zip_entries_delete -> zip_files_move -> zip_mem_move memmoves onto m_pMem to compact, before any write_func call
• the copy-on-write only guards the write/central-dir path, so this shift is unprotected

On a read-only backing (const/.rodata/PROT_READ mmap) that memmove is a write-through-const fault; on a writable buffer it silently mutates the caller's original bytes. Copy into a library-owned block on the first move, matching zip_stream_delete_write_func, so zip_stream_close frees it. Added test_entries_delete_stream_buffer_untouched (fails on master, passes here).