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/cbindgen.toml

@@ -0,0 +1,20 @@
+language = "C"
+header = "/* Auto-generated by cbindgen — do not edit. */"
+include_guard = "DREPORT_H"
+pragma_once = true
+no_includes = false
+sys_includes = ["stdint.h", "stddef.h", "stdbool.h"]
+cpp_compat = true
+documentation = true
+documentation_style = "doxy"
+style = "type"
+
+[export]
+prefix = ""
+
+[export.rename]
+"DreportHandle" = "DreportHandle"
+"DreportBuffer" = "DreportBuffer"
+
+[parse]
+parse_deps = false