Cover rename, partition-spec evolution, history retention; tighten queue tests

Three new tests:
- test_external_write_preserves_history_files: PyIceberg append after
  pg_lake insert. Old data files must NOT be queued for deletion (still
  referenced by the retained pre-append snapshot).
- test_external_write_partition_spec_evolution: PyIceberg adds an
  identity partition; new spec must register and reads must work
  across both specs.
- test_external_write_rename_column: PyIceberg renames a column;
  field IDs are stable so the sync must rename in pg_class, not
  silently drop+add.

The three deletion-queue tests are updated to assert the corrected
semantic: overwrite/empty-overwrite/append+overwrite all retain the
prior snapshot(s), so files stay referenced and are NOT queued for
deletion. (Physical-deletion-of-orphans coverage requires snapshot
expiration, which pyiceberg's Python API does not expose; that path
is exercised in pg_lake's internal tests already.)

Author: sfc-gh-mslot