Wrap cityhash in namespace and add accessor methods for metagraph com…

[adamant-pwn]

…patibility

Changes for metagraph integration:

1. Wrap cityhash in namespace to avoid typedef pollution

   • cityhash.hpp and cityhash.cpp: Wrapped in 'cityhash' namespace
   • Prevents conflicts with rollinghashcpp uint64 typedef
   • hash_util.hpp: Use cityhash::CityHash128WithSeed instead of static CityMurmur
   • tools/query.cpp: Changed uint64 to uint64_t (standard type)

2. Add public accessor methods to dictionary

   • Added kmer_type typedef for easier template parameter extraction
   • Added strings() accessor returning m_spss.strings bit vector
   • Added strings_offsets() accessor returning m_spss.strings_offsets
   • Moved strings_offsets() next to string_offsets() for better organization

3. Build system changes

   • CMakeLists.txt: Added cityhash.cpp to SSHASH_SOURCES
   • CMakeLists.txt: Link sshash executable against sshash_static (includes cityhash)

4. Update pthash submodule

   • Now tracking ratschlab/pthash master with compute_empirical_entropy fix
   • .gitmodules: Updated URL to point to ratschlab/pthash