ci: cache the NuGet package store across the .NET jobs (refs #73)

lint, build, unit and mutation each restored packages from the network on every
run. There are no lock files (so setup-dotnet's built-in cache doesn't apply), so
cache ~/.nuget/packages keyed on the project files via actions/cache. Pinned @v3
to avoid the GHES guard that breaks @v4 on Gitea (gitea-actions-gotchas.md); the
cache is best-effort, so a miss simply restores from the network.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -23,6 +23,16 @@ jobs:
       - uses: https://github.com/actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
+      # Cache the NuGet package store so each .NET job restores from disk, not the network. There are
+      # no lock files (so setup-dotnet's built-in cache doesn't apply); key on the project files. @v3
+      # avoids the GHES guard that breaks @v4 on Gitea (gitea-actions-gotchas.md); cache is best-effort
+      # — a miss just restores from the network. See issue #73.
+      - uses: https://github.com/actions/cache@v3
+        with:
+          path: ~/.nuget/packages
+          key: nuget-${{ runner.os }}-${{ hashFiles('**/*.csproj') }}
+          restore-keys: |
+            nuget-${{ runner.os }}-
       - run: make lint
 
   build:
@@ -32,6 +42,12 @@ jobs:
       - uses: https://github.com/actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
+      - uses: https://github.com/actions/cache@v3
+        with:
+          path: ~/.nuget/packages
+          key: nuget-${{ runner.os }}-${{ hashFiles('**/*.csproj') }}
+          restore-keys: |
+            nuget-${{ runner.os }}-
       - run: make build
 
   unit:
@@ -41,6 +57,12 @@ jobs:
       - uses: https://github.com/actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
+      - uses: https://github.com/actions/cache@v3
+        with:
+          path: ~/.nuget/packages
+          key: nuget-${{ runner.os }}-${{ hashFiles('**/*.csproj') }}
+          restore-keys: |
+            nuget-${{ runner.os }}-
       - run: make unit
 
   # Frontend (Nx/Angular) lane: install with pnpm, then Nx lint + test + build.
@@ -64,6 +86,12 @@ jobs:
       - uses: https://github.com/actions/setup-dotnet@v4
         with:
           dotnet-version: '10.0.x'
+      - uses: https://github.com/actions/cache@v3
+        with:
+          path: ~/.nuget/packages
+          key: nuget-${{ runner.os }}-${{ hashFiles('**/*.csproj') }}
+          restore-keys: |
+            nuget-${{ runner.os }}-
       - run: make mutation
       # Publish the Stryker HTML reports. `if: always()` uploads them even when the
       # ratchet fails — that is exactly when you want to inspect the survivors.