Replace AutoCompact._archive() direct session mutation with delegation
to Consolidator.compact_idle_session(). Remove _split_unconsolidated()
method since that logic now lives inside compact_idle_session.
All session mutation for idle compaction now goes through the
Consolidator's lock, eliminating the race condition between
background token consolidation and idle TTL compaction.
Changes:
- autocompact.py: rewrite _archive() to call compact_idle_session,
remove _split_unconsolidated(), clean up unused imports
- test_autocompact_unit.py: replace TestArchive/TestSplitUnconsolidated
with TestArchiveDelegates that verifies delegation behavior
- test_auto_compact.py: convert all consolidator.archive mocks to
consolidator.compact_idle_session mocks via _make_fake_compact helper
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