Add nuget v3 remote type

- add `nuget` package type proxying a NuGet v3 feed, with `index.json` mapped to the upstream service index
- rewrite upstream-origin URLs in served JSON per request so flat container and registration lookups go through the remote
- treat `.nupkg`/`.nuspec` as immutable and indexes as mutable
- add a `nuget.config` usage snippet to the UI and a nuget e2e caching case

ui/src/components/UsageInstructions.tsx

@@ -243,6 +243,22 @@ index = "sparse+${proxy}/"`,
         },
       ];
 
+    case 'nuget':
+      return [
+        {
+          title: 'Use this feed instead of nuget.org',
+          language: 'xml',
+          code: `<!-- nuget.config -->
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="${name}" value="${proxy}/index.json" protocolVersion="3" />
+  </packageSources>
+</configuration>`,
+          note: 'Serve artifactapi over HTTPS: NuGet rejects the repository signatures resource over plain HTTP.',
+        },
+      ];
+
     case 'puppet':
       return [
         {

ui/src/pages/Locals.tsx

@@ -18,6 +18,7 @@ const typeColors: Record<string, 'blue' | 'green' | 'yellow' | 'red' | 'default'
   terraform: 'blue',
   goproxy: 'green',
   cargo: 'yellow',
+  nuget: 'blue',
 };
 
 export function Locals() {

ui/src/pages/Remotes.tsx

@@ -18,6 +18,7 @@ const typeColors: Record<string, 'blue' | 'green' | 'yellow' | 'red' | 'default'
   terraform: 'blue',
   goproxy: 'green',
   cargo: 'yellow',
+  nuget: 'blue',
 };
 
 export function Remotes() {