fix(db): persist delete-only writing segment records

[egolearner]

Problem

A writing segment can contain delete WAL records while its document count remains zero. Schema and index changes previously used the document count to decide whether to switch the writing segment, so a delete-only WAL could be destroyed before its delete snapshot was persisted. This could make collection recovery fail or allow deleted documents to reappear.

Fix

• Add Segment::has_record() to account for both documents and WAL records.
• Use has_record() for schema, index, and optimize switch decisions.
• Flush delete-only writing segments without creating empty persisted segments.
• Add a reopen regression test covering column DDL after a delete-only write.

Tests

• ninja -C build.release unittest.collection_test
• 83 tests passed.