Implement Economic Attention Network (ECAN) for memory importance management:
- EcanParams: decay_rate, beta, spread_factor, threshold
- decay_sti(): STI decay toward zero over time
- update_lti(): LTI accumulation weighted by STI, penalized below threshold
- spread(): boost STI for confirmed memories
- cycle(): full decay + LTI update weighted by truth value
- initialize(): compute initial STI/LTI from truth value and confidence
14 unit tests covering decay convergence, LTI growth/penalty,
spread clamping, cycle behavior, initialization, and bounds.
Part of #29
