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>

dreport-ffi/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "dreport-ffi"
+version = "0.2.0"
+edition = "2024"
+description = "C ABI for dreport-service (consumed by NuGet, native hosts, etc.)"
+license = "MIT"
+publish = false
+
+[lib]
+crate-type = ["cdylib", "rlib", "staticlib"]
+
+[dependencies]
+dreport-service = { path = "../dreport-service" }
+serde_json = "1"
+
+[build-dependencies]
+cbindgen = { version = "0.28", default-features = false }