Run library unit tests in check pipeline

The nextest filter `--bin mdkd` only compiles tests under the `mdkd`
binary target and skips every `#[cfg(test)] mod tests` in the `mdk`
library crate (src/lib.rs and its module tree). Switching to
`--lib --bin mdkd` adds the lib tests without pulling in
`tests/integration.rs`, which needs VSS + Postgres and runs under
`just integration-test` instead.

Author: amackillop

flake.nix

@@ -195,7 +195,7 @@
             commonArgs
             // {
               inherit cargoArtifacts;
-              cargoNextestExtraArgs = "--bin mdkd";
+              cargoNextestExtraArgs = "--lib --bin mdkd";
             }
           );
         };

justfile

@@ -94,9 +94,9 @@ integration-test *args:
 fix:
     cargo clippy --all-targets --fix --allow-dirty --allow-staged
 
-# Run tests (unit + doc; use `just integration-test` for integration tests)
+# Run tests (lib + bin unit tests; use `just integration-test` for integration tests)
 test *args:
-    cargo nextest run --bin mdkd {{args}}
+    cargo nextest run --lib --bin mdkd {{args}}
 
 # Run the server
 run *args: