test(omo-senpi): pin memory_notice to the messages compaction removed

The notice claims "N previous messages ... have left the live context" with N
taken from the branch length, so a session that never compacted reports its own
live entries as gone. These tests read the branch the way senpi does: only the
message entries positioned before the latest compaction's firstKeptEntryId (or
before the compaction entry when that id is no longer on the branch) have left
the live context, an uncompacted branch reports nothing, and a notice with no
lines at all injects no message.

The standing fact that recalled memory arrives on its own and has no tool to
call moves into memory-core's compiled reminder, where it is pinned once.