add ptrhash implementation for 2.6

[sparknack]

• Add a header-only PtrHash implementation with integer, string, string_view, prehashed-key, serialization, and mmap loading support.
• Add correctness and stability coverage for minimal permutation guarantees, integer/string corner cases, duplicate rejection, corrupt serialized layouts, remap bounds, mmap move/error paths, non-member query bounds, concurrent query stability, and mutated serialized input.
• Tighten deserialization and empty-input parameter validation so invalid data fails before it can produce out-of-range results.
• Add explicit mmap cleanup on failed mapped-open validation and test that corrupt mmap input leaves no mapped file behind on Linux.