Fix cs md strlen overrun - #168

minimap2 C code calls strlen() on the seq pointer as the first operation in mm_gen_cs_or_MD(). Rust &[u8] slices are not NUL-terminated, so passing seq.as_ptr() directly causes __strlen_avx2 SIMD reads to walk past the
buffer into unmapped pages, triggering SIGSEGV (error 4).

Fix: build a NUL-terminated copy (seq_nul) once per read in the cs||md block and pass seq_nul_ptr to all mm_gen_cs/mm_gen_MD calls.

Also remove redundant CStr::from_ptr(contig.as_ptr()) that re-strlen an already-known-length CStr.

Tested on long-read scNanoATAC-seq data (13.8M reads, exit 0).