learning_mutations re-implemented the §-delimited read/write that
tools/memory_tool already owns, and its writer used a plain write_text
(truncate-then-write) — reintroducing exactly the partial-file race that
MemoryStore._write_file engineered away with atomic temp-file + rename.
Reuse MemoryStore._read_file/_write_file so the format is single-sourced,
the write is atomic against concurrent readers, and journey indices stay
aligned with the graph.