rebuild

Discards every sidecar and rebuilds them from the segments, keeping the index definitions.

The escape hatch that makes this layer's relaxed attitude to sidecar durability defensible: a posting list is derived, so damage costs a scan rather than data. The definitions are not derived and are deliberately kept — losing those to a repair would turn a corrupt file into a lost instruction, which is the one thing the registry's durability rule exists to prevent.