test: add reranker edge case coverage for RRF and Weighted re-rankers (closes #102)

[s2x]

Description

Closes #102

Changes

• Added 7 .phpt test files for reranker edge cases:
  • test_reranker_rrf_empty.phpt — empty query results, null values, non-ZVecDoc elements
  • test_reranker_weighted_zero_range.phpt — zero-range normalization guard (all scores identical)
  • test_reranker_weighted_float_min_bug.phpt — PHP_FLOAT_MIN initialization bug detection
  • test_reranker_weighted_l2_metric.phpt — L2 metric normalization (lower distance = higher score)
  • test_reranker_weighted_negative_scores.phpt — negative IP score normalization
  • test_reranker_rrf_custom_rank_constant.phpt — custom rank constant effect on combined scores
  • test_reranker_weighted_empty_weights.phpt — empty weights edge cases (constructor + setter)
• Added test_reranker_in_query.phpt — integration test for queryWithReranker() with RRF
• Fixed 4 test EXPECT sections: EXPECTF vs EXPECT usage, format specifiers
• Fixed PHP_FLOAT_MIN bug detection logic in test_reranker_weighted_float_min_bug.phpt
• Updated CHANGELOG.md with TEST-004 entry

Testing

• All 9 reranker .phpt tests pass (100%)
• Each test uses try-finally with uniqid() temp directory and cleanup
• Code review via subagent passed (all issues addressed)

Code Review

• Passed subagent code review
• All review comments addressed (4 issues fixed)