feat: add memory leak regression tests (#105)

[s2x]

Summary

Adds memory leak regression tests for FFI memory safety as specified in #105.

Changes

• 5 new .phpt test files for memory leak detection:
  • test_memory_collection_lifecycle.phpt - 50x create/open/close/destroy cycle
  • test_memory_deserialize_buffer.phpt - 100x serialize/deserialize cycle
  • test_memory_query_output_fields.phpt - 100x query with/without output fields
  • test_memory_init_error_path.phpt - 20x init error/recovery cycles
  • test_memory_delete_cstrings.phpt - 30x insert/delete/fetch cycles
• New example: examples/07_memory_management.php - FFI memory cleanup patterns with VmRSS monitoring
• Documentation: Updated CHANGELOG.md and AGENTS.md with memory leak regression test section

Testing

• All 143 tests pass (140 PASS, 1 skipped, 2 expected fail)
• Each test uses try-finally with uniqid() temp directory under test_dbs/
• Tests verify both PHP heap and native memory stability with 500KB threshold

Closes

Closes #105