refactor: modularize checks into separate Nix files

- Extracted checks (clippy, doc, fmt, and toml-fmt) into modularized Nix files under `nix/checks`.
- Updated `flake.nix` to import checks from the new modularized structure.
- Improved clarity and maintainability by separating concerns for each check.

nix/checks/clippy.nix

@@ -0,0 +1,13 @@
+{
+  craneLib,
+  commonArgs,
+  cargoArtifacts,
+}:
+
+craneLib.cargoClippy (
+  commonArgs
+  // {
+    inherit cargoArtifacts;
+    cargoClippyExtraArgs = "--all-targets -- --deny warnings";
+  }
+)

nix/checks/default.nix

@@ -0,0 +1,30 @@
+{
+  craneLib,
+  src,
+  commonArgs,
+  cargoArtifacts,
+  vault-hier,
+  pkgs,
+}:
+
+{
+  inherit vault-hier;
+
+  my-workspace-clippy = import ./clippy.nix {
+    inherit craneLib commonArgs cargoArtifacts;
+  };
+
+  my-workspace-doc = import ./doc.nix {
+    inherit craneLib commonArgs cargoArtifacts;
+  };
+
+  # Check formatting
+  my-workspace-fmt = import ./fmt.nix {
+    inherit craneLib src;
+  };
+
+  my-workspace-toml-fmt = import ./toml-fmt.nix {
+    inherit craneLib src;
+    lib = pkgs.lib;
+  };
+}

nix/checks/doc.nix

@@ -0,0 +1,12 @@
+{
+  craneLib,
+  commonArgs,
+  cargoArtifacts,
+}:
+
+craneLib.cargoDoc (
+  commonArgs
+  // {
+    inherit cargoArtifacts;
+  }
+)

nix/checks/fmt.nix

@@ -0,0 +1,5 @@
+{ craneLib, src }:
+
+craneLib.cargoFmt {
+  inherit src;
+}

nix/checks/toml-fmt.nix

@@ -0,0 +1,11 @@
+{
+  craneLib,
+  src,
+  lib,
+}:
+
+craneLib.taploFmt {
+  src = lib.sources.sourceFilesBySuffices src [ ".toml" ];
+  # taplo arguments can be further customized below as needed
+  # taploExtraArgs = "--config ./taplo.toml";
+}