v0.0.7 — POSIX fd hashing and opt-in mmap fast path

Highlights

• Added POSIX fdHash / zfh_fd_hash for wrappers that already have an open
  file descriptor but do not have a usable filesystem path. This covers
  Android content:// providers, security-scoped URLs, provider-backed URLs,
  and other platform APIs that expose files only as file descriptors. The path
  API cannot represent these sources reliably without copying them to a
  temporary file.
• The fd API keeps the read loop inside Zig, avoids copying the source to a
  temporary file or calling into Zig for every chunk, reads from the file
  descriptor's current position, and never closes it. It remains a streaming
  implementation and intentionally does not use mmap.
• Added an opt-in mmap fast path for fileHash / fileHashInDir and
  zfh_context_file_hash. Enable it with Zig's HashRequest.use_mmap or the
  C flag ZFH_OPTION_USE_MMAP when hashing a stable regular file.
• mmap is disabled by default. It is a file-I/O optimization, not a different
  hashing algorithm: the digest stays identical while the input is processed
  through a memory mapping and 64 KiB chunks. Callers should benchmark their
  workload before enabling it; local measurements were usually within a few
  percent, ranging from slightly slower to about 20% faster. The mapped file
  must not be modified or truncated while it is being hashed.

Compatibility notes

• C ABI version is now ZFH_API_VERSION = 4.
• ZFH_OPTION_USE_MMAP does not change the layout of zfh_options.
• The mmap option affects path-based file hashing only; it is ignored by
  zfh_fd_hash and the streaming hasher APIs.