bluememoStrength
How much a memory has been needed, which decides what survives when the store is full.
storage_strength starts at 1, or 2 when the person asked for the memory to be kept, and only ever grows. Retrievability is computed from it rather than stored: it halves every HalfLife × storage_strength since the memory was last recalled. Usefulness is their product, and it orders eviction. It never orders search results.