Fix Secret Key decode rejecting valid keys; verify build

The first base32 symbol carries 3 data bits (26 symbols = 130 bits, 2 spare),
but decode only accepted values 0-3, so ~half of generated keys failed to parse.
Adds an edge-case test, rustfmt config, Cargo.lock, and on-demand cross targets
instead of installing all seven in rust-toolchain.toml.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/kv-store/src/lib.rs

@@ -27,7 +27,9 @@ pub enum StoreError {
 pub enum SyncState {
     Clean,
     /// Edited locally; `base_revision` is what the edit was made against.
-    Pending { base_revision: u64 },
+    Pending {
+        base_revision: u64,
+    },
 }
 
 /// Storage backend. The SQLite implementation and an in-memory one for tests come in M1.