[s2x]

What changed

PHPStan analysed at PHP 8.4 although the package floor is PHP 8.2, so code
using PHP 8.3/8.4 functions could pass despite the declared minimum. The
PHP 8.2 unit job also never loaded the real libfdb_c.

• phpstan.neon: target is now phpVersion: 80200, includes
  phpstan-baseline.neon, and documents the PHP <= 8.3 FFI-stub bug
  (FFI::new()/FFI::cast() typed ?FFI\CData; fixed in PHP 8.4, php-src
  a37a3d3d81). No null handling is added to the wrappers.
• phpstan-baseline.neon (new): only the 73 FFI-stub nullability false
  positives. The two stale property.notFound ignores were removed.
• .github/workflows/ci.yml: the FoundationDB client is installed for every
  unit-matrix entry, so the real-library path runs on PHP 8.2/8.3/8.4.
• tests/Unit/NativeClientLibraryPathTest.php: docblock updated from "the
  PHP 8.4 CI job" to the full matrix; CHANGELOG.md records the change.

Closes #164

How it was tested

• vendor/bin/phpstan analyse --memory-limit=512M --no-progress at
  phpVersion: 80200: no errors. The raw pre-baseline list was exactly 73
  errors, all FFI-stub nullability; no real 8.2 incompatibility.
• PHP 8.2.35 container with libfdb_c 7.3.75:
  NativeClientLibraryPathTest 8/8 passed (real-library load, not skipped);
  full suite 919 tests, 2480 assertions, 6 skipped.
• PATH=/tmp/opencode/bin:$PATH bin/lint.sh, actionlint, git diff --check
  pass.

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 (chore: ...)