teknium1 review on #57690: harvesting logic was skipping the ENTIRE merged
row when a compaction summary was appended to the tail message, discarding
real prior user content that context_compressor retains before the
_MERGED_SUMMARY_DELIMITER. Extract and harvest that pre-delimiter segment
instead of dropping it wholesale.
Revert-to-fail: reverting plugins/memory/holographic/__init__.py alone
drops test_merged_into_tail_preserves_genuine_pre_delimiter_preference
(19 passed, 1 failed); restoring the fix returns 20/20 passed.
