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/build.rs

@@ -0,0 +1,46 @@
+//! Generates `include/dreport.h` from the public C ABI on every build.
+//! Header is checked into the repo so consumers (NuGet wrapper, manual C use)
+//! don't need a Rust toolchain.
+
+use std::env;
+use std::path::PathBuf;
+
+fn main() {
+    println!("cargo:rerun-if-changed=src/lib.rs");
+    println!("cargo:rerun-if-changed=cbindgen.toml");
+    println!("cargo:rerun-if-changed=build.rs");
+
+    // Skip generation if explicitly disabled (e.g. when cross-compiling without
+    // host tools, or in cargo publish dry runs).
+    if env::var("DREPORT_FFI_SKIP_CBINDGEN").is_ok() {
+        return;
+    }
+
+    let crate_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
+    let out_path = PathBuf::from(&crate_dir).join("include").join("dreport.h");
+    if let Some(parent) = out_path.parent() {
+        let _ = std::fs::create_dir_all(parent);
+    }
+
+    let config_path = PathBuf::from(&crate_dir).join("cbindgen.toml");
+    let config = if config_path.exists() {
+        cbindgen::Config::from_file(&config_path).expect("invalid cbindgen.toml")
+    } else {
+        cbindgen::Config::default()
+    };
+
+    match cbindgen::Builder::new()
+        .with_crate(crate_dir)
+        .with_config(config)
+        .generate()
+    {
+        Ok(bindings) => {
+            bindings.write_to_file(&out_path);
+        }
+        Err(e) => {
+            // Don't fail the build on header generation problems — the cdylib
+            // is still usable; the header is a developer convenience.
+            println!("cargo:warning=cbindgen header generation failed: {}", e);
+        }
+    }
+}