[s2x]

What changed

NativeClient loaded libpthread.so.0 and libdl.so.2 unconditionally, so
NativeClient::getInstance() failed on macOS before the FDB library was usable.

• src/NativeClient.php: platform-aware pthread/dl selection — libSystem.B.dylib
  on Darwin, libpthread.so.0/libdl.so.2 on Linux, a clear
  RuntimeException elsewhere — and a platform-aware default soname
  (libfdb_c.dylib on macOS).
• tests/Unit/NativeClientPlatformLibrariesTest.php (new): Darwin/Linux
  mappings, unsupported OS rejection and default soname, without needing macOS.
• tests/Unit/NativeClientLibraryPathTest.php: reads FDB_LIBRARY_PATH first,
  adds Homebrew//usr/local/lib//usr/lib .dylib and Linux lib64 paths,
  uses a platform-neutral skip message, and saves/restores the ambient
  FDB_LIBRARY_PATH instead of unsetting it.
• docs/getting-started.md / README.md document the macOS library name and
  FDB_LIBRARY_PATH; CHANGELOG.md records the fix.

Refs #165 — the code is merged, but the macOS runtime load is still a manual
check (no macOS runner here), so the issue stays open until that is done.

How it was tested

• PHP 8.2 container: Unit suite 617 tests, 1267 assertions; full suite
  926 tests, 2500 assertions, 6 skipped; Integration 309 tests, 1233 assertions, 6 skipped.
• Unit suite with an ambient FDB_LIBRARY_PATH: the value survives the suite.
• Linux mapping and default soname are byte-identical to before;
  resolveLibraryPath() precedence is unchanged.
• PHPStan, PHP-CS-Fixer, Rector and git diff --check pass.
• macOS runtime was not available; the Darwin mapping is covered by unit tests
  and inspected against libSystem/dyld semantics, but the real load remains a
  manual/macOS-CI check.

Checklist

• Tests added or updated, and all checks pass
• CHANGELOG.md updated under [Unreleased]
• Documentation updated
• Everything is written in English
• The PR title is a Conventional Commit (fix: ...)