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>

backend/Cargo.toml

@@ -5,12 +5,14 @@ edition = "2024"
 publish = false
 
 [dependencies]
-dreport-core = { path = "../core" }
-dreport-layout = { path = "../layout-engine" }
+dreport-service = { path = "../dreport-service" }
 axum = "0.8"
 tokio = { version = "1", features = ["full"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tower-http = { version = "0.6", features = ["cors"] }
-thiserror = "2"
 anyhow = "1"
+
+[dev-dependencies]
+tower = { version = "0.5", features = ["util"] }
+http-body-util = "0.1"