fix: a refused detach no longer starts the drop grace

[AlexeyShalaev]

Closes #105.

Two changes, each enough on its own for the case in the issue. Together they also keep an attached partition from carrying a comment that says it was detached.

• _mark_orphaned keeps an existing detached-at only while the table is detached or has a detach pending (pg_inherits.inhdetachpending). On a partition that is still plainly attached, the instant is a leftover, so the detach records its own (orphan_comment(..., keep_instant=False)). The unpinned concurrent path no longer skips tables that already carry a marker; it goes through the same rule.
• _unmark_if_refused: after a 55006 or 23503 refusal on either concurrent path, the marker lines come off and the comment the table had before stays. This runs under SHARE UPDATE EXCLUSIVE on the partition, which is what COMMENT takes anyway, checked against the expected OID, and only while the partition is still plainly attached. It is best effort: if it fails, a warning is logged, the refusal is still what the run reports, and the stamp rule keeps a leftover harmless. 42P01 is left alone, since there is no table to fix.

The test (both mirrors) uses a June partition held by a foreign key, with a comment of its own, and DropAfter(grace=30 days):

1. A run on 2026-08-26 is refused; afterwards June's comment is still exactly june archive.
2. The reference is deleted, and a run on 2026-08-28 detaches June.
3. Its stamp is 2026-08-28 and the comment ends with june archive.

Without the change the first assertion fails: the comment still carries the marker.

Results: unit 2872 passed, integration (PG 17) 579 passed, ruff and mypy clean. sync_tests_mirror.py also rewrote three unrelated files (the known drift); I reverted those and kept only test_foreign_keys.py.

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pg_partsmith/aio/repositories/remover.py	62.06%	9 Missing and 2 partials ⚠️
pg_partsmith/sync/repositories/remover.py	83.33%	3 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!