feat: dreport-service + dreport-ffi + nuget packages

Extract orchestration (font registry + render pipeline) from the Axum
backend into a standalone dreport-service crate. Backend becomes a thin
HTTP adapter on top.

Add dreport-ffi (cdylib) exposing the service through a stable C ABI
with opaque handles, byte buffers, and thread-local error reporting.

Build Dreport.Service + Dreport.AspNetCore NuGet packages under
bindings/dotnet/, packing the host RID native binary via a generated
nuspec. justfile recipes (nuget-publish, nuget-publish-all) build,
pack, and push to the Gitea NuGet registry in one shot.

Test coverage: 47 Rust + 38 C# (xUnit + WebApplicationFactory).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -9,3 +9,17 @@ dist/
 frontend/tests/visual/cross-renderer-refs/
 frontend/tests/visual/cross-renderer-diffs/
 frontend/tests/visual/test-results/
+
+# .NET build artifacts
+**/bin/
+**/obj/
+
+# Native runtime binaries — produced by `just nuget-build-native-*`
+# and packaged into the .nupkg. Never commit.
+bindings/dotnet/src/Dreport.Service/runtimes/
+
+# Auto-generated nuspec (regenerated by justfile recipes)
+**/.generated.nuspec
+
+# Generated C header (regenerated on every dreport-ffi build)
+dreport-ffi/include/