Update CHECKPOINT-MEM.md to reflect that Steps 1-9 of
lets-see-options-for-merry-dijkstra.md are shipped. Notes that
ISSUE-MEM-002 is closed in the user-call path (matchContextAllocCall
remains as a documented comptime escape hatch), Phase 1.2/1.3/1.4
are unblocked, and points future sessions at the MEM plan
(tidy-doodling-cray.md) for the next phase.
