Fix duplicate keys in RangeResult reverse pagination (#73)

[s2x]

Summary

RangeResult reverse pagination yielded each boundary key twice when a
range span more than one server page. The iterator advanced the upper
bound at every batch boundary with KeySelector::firstGreaterOrEqual,
an inclusive selector, so the last key returned in batch N was
returned again as the first key in batch N+1.

The forward branch already uses firstGreaterThan and is unaffected;
this PR makes the reverse branch do the same.

Reproducer (before the fix)

$result = $db->getRangeStartsWith('p/', new RangeOptions(reverse: true));
// $result has duplicates — every page-seam key appears twice.

Fix

src/RangeResult.php — flip the reverse branch from
KeySelector::firstGreaterOrEqual() to
KeySelector::firstGreaterThan(), mirroring the forward
branch.

Tests

tests/Integration/RangeReadTest.php — two new tests over a 1000-key,
~50-byte value range (forces multiple server pages):

1. getRangeReversePaginationYieldsEachKeyOnce — full reverse range,
   asserts count == 1000, first is p/0999, last is p/0000,
   no duplicates, exact ordering.
2. getRangeReversePaginationWithLimitAcrossBatches — limit: 750 on
   the same range size, asserts count == 750, no duplicates,
   last-key == first of new batch.

Both tests fail under the old behavior and pass after the fix.

Checklist

• composer test:unit — 429 tests, 874 assertions, OK
• composer cs — clean
• composer rector — clean (also applied SortCallLikeNamedArgsRector)
• composer phpstan — clean
• composer test:integration — local environment has no FDB
  cluster; covered by CI against docker-compose up
• CHANGELOG updated under Unreleased / Fixed with the [[Bug] RangeResult pagination yields duplicate keys when iterating in reverse #73] anchor
• New branch fix/issue-73-rangeresult-reverse-duplicates from
  master