ci: add distcheck for lib-vt source tarball

Add a build-dist-lib-vt job that runs distcheck with
-Demit-lib-vt=true and verifies the resulting tarball stays under
5 MB. Also downsize the build-dist runner from -md to -sm.

.github/workflows/test.yml

@@ -85,6 +85,7 @@ jobs:
       - skip
       - build-bench
       - build-dist
+      - build-dist-lib-vt
       - build-examples-zig
       - build-examples-cmake
       - build-examples-cmake-windows
@@ -652,7 +653,7 @@ jobs:
         run: nm result/bin/.ghostty-wrapped 2>&1 | grep -q 'main_ghostty.main'
 
   build-dist:
-    runs-on: namespace-profile-ghostty-md
+    runs-on: namespace-profile-ghostty-sm
     needs: test
     outputs:
       artifact-id: ${{ steps.upload-artifact.outputs.artifact-id }}
@@ -693,6 +694,48 @@ jobs:
           path: |-
             ghostty-source.tar.gz
 
+  build-dist-lib-vt:
+    runs-on: namespace-profile-ghostty-sm
+    needs: test
+    env:
+      ZIG_LOCAL_CACHE_DIR: /zig/local-cache
+      ZIG_GLOBAL_CACHE_DIR: /zig/global-cache
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Setup Cache
+        uses: namespacelabs/nscloud-cache-action@a90bb5d4b27522ce881c6e98eebd7d7e6d1653f9 # v1.4.2
+        with:
+          path: |
+            /nix
+            /zig
+
+      # Install Nix and use that to run our tests so our environment matches exactly.
+      - uses: cachix/install-nix-action@51f3067b56fe8ae331890c77d4e454f6d60615ff # v31.10.2
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - uses: cachix/cachix-action@1eb2ef646ac0255473d23a5907ad7b04ce94065c # v17
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
+      - name: Build and Check Source Tarball
+        run: |
+          rm -rf zig-out/dist
+          nix develop -c zig build distcheck -Demit-lib-vt=true
+
+      - name: Verify tarball size
+        run: |
+          tarball=$(ls zig-out/dist/*.tar.gz)
+          size=$(stat --format=%s "$tarball")
+          max=$((5 * 1024 * 1024))
+          echo "Tarball size: $size bytes (max: $max)"
+          if [ "$size" -gt "$max" ]; then
+            echo "ERROR: tarball exceeds 5 MB"
+            exit 1
+          fi
+
   trigger-snap:
     if: github.event_name != 'pull_request'
     runs-on: namespace-profile-ghostty-xsm