build: split nix dev shell into default and presets

Problem: the single dev shell mixed dev tooling (linters, test runner)
with preset compiler tools, causing heavy rebuilds (e.g. Chromium for
`mermaid-cli`) for contributors who only need the dev tools.

Solution: extract dev tooling into a shared `devTools` list and expose
two shells — `default` for development and `presets` for running all
built-in preset compilers (`typst`, `texliveMedium`, `tectonic`,
`pandoc`, `asciidoctor`, `quarto`, `plantuml`, `mermaid-cli`).

flake.nix

@@ -19,9 +19,9 @@
     {
       formatter = forEachSystem (pkgs: pkgs.nixfmt-tree);
 
-      devShells = forEachSystem (pkgs: {
-        default = pkgs.mkShell {
-          packages = [
+      devShells = forEachSystem (pkgs:
+        let
+          devTools = [
             (pkgs.luajit.withPackages (
               ps: with ps; [
                 busted
@@ -32,9 +32,24 @@
             pkgs.stylua
             pkgs.selene
             pkgs.lua-language-server
-            pkgs.plantuml
           ];
-        };
-      });
+        in
+        {
+          default = pkgs.mkShell {
+            packages = devTools;
+          };
+          presets = pkgs.mkShell {
+            packages = devTools ++ [
+              pkgs.typst
+              pkgs.texliveMedium
+              pkgs.tectonic
+              pkgs.pandoc
+              pkgs.asciidoctor
+              pkgs.quarto
+              pkgs.plantuml
+              pkgs.mermaid-cli
+            ];
+          };
+        });
     };
 }