fsck: add store integrity check and repair command

Reads each capnp log message sequentially, validates framing and
content. On first corrupt message, truncates to last good position
and removes stale caches so next load replays from repaired log.

Wired up as `poc-memory fsck`.

src/store/mod.rs

@@ -29,6 +29,7 @@ mod ops;
 pub use types::*;
 pub use parse::{MemoryUnit, parse_units};
 pub use view::{StoreView, AnyView};
+pub use persist::fsck;
 
 use crate::graph::{self, Graph};