[#68] Document Murmur3 64-bit seed semantics and C++ interoperability

[peter-lawrey]

What changed

• Document that murmur_3(long) uses all 64 bits of its seed.
• Define C++ interoperability in terms of the reference implementation's
  uint32_t seed and the same input byte sequence.
• Explain how to zero-extend a signed Java int with
  Integer.toUnsignedLong.
• Document the contract on both the 64-bit and 128-bit public APIs.
• Add C++ reference vectors at the 0x80000000 and 0xffffffff boundaries,
  checking both output words and the 64-bit API's first-word result.

Why

The C++ MurmurHash3_x64_128 reference accepts a uint32_t seed, while this
library has historically accepted and used a full Java long. Masking the
existing parameter would change established hash values, including potentially
persisted values, so this PR clarifies the contract without changing behaviour.

An int overload is deliberately not added: existing source calls such as
murmur_3(-1) currently bind to the long overload. A new overload with
zero-extension semantics would silently change their result after recompilation
while existing binaries retained the old result.

Validation

• Java 8: mvn -B clean verify
  • 14,904 tests, 0 failures, 0 errors, 12 skipped
  • Checkstyle, Javadocs and licence checks passed
• Independently generated and verified the two boundary vectors with Austin
  Appleby's C++ MurmurHash3_x64_128 reference implementation
• git diff --check

Fixes #68