v0.3.0 - SECDED RAS verification

Adds an optional data-SECDED structural variant while preserving the parity baseline. The release includes single-bit correction and scrub, double-bit containment, corrected dirty writeback/maintenance behavior, an independent C++ SECDED model, named assertions, and a 7/7 RAS coverage matrix. Core evidence remains 22/22 directed tests, 100/100 stress scenarios, 127/127 trace replays, 55/55 interaction crosses, and 4/4 mutation detections.