Add configurable I/O buffer via Chksumer context

[ventaquil]

Introduces Chksumer/AsyncChksumer checksum contexts with a configurable
read-buffer capacity, routing all sync and async I/O through a single reusable
buffer. Fixes GH-2.

Changes

• New context API — Chksumer/AsyncChksumer + builders with configurable
  capacity (with_capacity, DEFAULT_BUFFER_CAPACITY), the update/update_from/
  update_from_reader feed family, reset, into_inner, From<H> conversions,
  and an io::Write impl.
• Renames — Hashable::hash_with → hash_into, chksum_with → chksum_into.
• NotARegularFile error variant; directory traversal now skips non-regular
  entries (sockets, fifos) instead of trying to open them.
• Toolchain — edition 2024, MSRV 1.95.0, thiserror 2.0; CI matrix bumped and
  the three per-OS build jobs collapsed into one os × toolchain matrix.
• Docs — symlink traversal behavior documented; # Errors sections added to all
  public Result-returning functions.

Notes

• Breaking API changes; remains unreleased under CHANGELOG [Unreleased] (version
  stays 0.1.0 until a separate release commit).
• Every commit builds under --all-features; fmt/clippy -D warnings/tests all green.

[ventaquil]

Pre-publish check found several bugs - I'll fix them later.