build: distribute gresource c/h with source tarball

This introduces the concept of a "dist resource" (specifically a
`GhosttyDist.Resource` type). This is a resource that may be present in
dist tarballs but not in the source tree. If the resource is present and
we're not in a Git checkout, then we use it directly instead of
generating it.

This is used for the first time in this commit for the gresource c/h
files, which depend on a variety of external tools (blueprint-compiler,
glib-compile-resources, etc.) that we do not want to require downstream
users/packagers to have and we also do not want to worry about them
having the right versions.

This also adds a check for `distcheck` to ensure our distribution
contains all the expected files.

.github/workflows/test.yml

@@ -239,7 +239,17 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: Build and Check Source Tarball
-        run: nix develop -c zig build distcheck
+        run: |
+          rm -rf zig-out/dist
+          nix develop -c zig build distcheck
+          cp zig-out/dist/*.tar.gz ghostty-source.tar.gz
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: source-tarball
+          path: |-
+            ghostty-source.tar.gz
 
   build-macos:
     runs-on: namespace-profile-ghostty-macos
@@ -828,7 +838,7 @@ jobs:
   test-debian-12:
     name: Test build on Debian 12
     runs-on: namespace-profile-ghostty-sm
-    needs: test
+    needs: [test, build-dist]
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -839,6 +849,11 @@ jobs:
       - name: Configure Namespace powered Buildx
         uses: namespacelabs/nscloud-setup-buildx-action@v0
 
+      - name: Download Source Tarball Artifacts
+        uses: actions/download-artifact@v4
+        with:
+          name: source-tarball
+
       - name: Build and push
         uses: docker/build-push-action@v6
         with: