Import december-2020-12-18 from private repo (scrubbed 2026-05-27)

Targets Rec Room 2020.12.18 client.

Re-import after adding:
- DorkNet.ClientMod/dorknet-clientmod.json.template — config schema
  the launcher's manifest-driven patcher renders into UserData/.
- tools/build-patcher-zip.ps1 — produces the dorknet-clientpatch-{key}.zip
  Release asset the launcher fetches.
- .github/workflows/release.yml — auto-publishes both Release assets
  on v1-december-* tag push.

Scrubbed via maintainer-side scrub-for-public.ps1: same drop rules as
the prior import, plus build-patcher-zip.ps1 added to the tools/
allow-list so it survives the scrub.

DorkNet.Server/Versions/Late2020/Late2020VersionPlugin.cs

@@ -0,0 +1,42 @@
+using DorkNet.Server.Compat;
+
+namespace DorkNet.Server.Versions.Late2020;
+
+/// <summary>The "Late 2020" wire-format generation: covers all Rec Room
+/// builds from the autumn 2020 schema migration through the end of the
+/// year. The 2020.12.18 build is the canonical member; if/when we add
+/// 2020.10 or 2020.11 builds and find their wire shapes are identical,
+/// we just list them in <see cref="VersionKeys"/> here — no new code.
+///
+/// <para>Currently this plugin is a marker: it declares which version
+/// keys belong to the generation, but no strategy services are
+/// registered yet. As controllers start branching on
+/// <see cref="HttpContextExtensions.ClientVersion"/>'s
+/// <c>Generation</c>, the strategies that get extracted will register
+/// themselves through <see cref="RegisterStrategies"/> with a
+/// keyed-DI binding so the right implementation resolves per
+/// request.</para>
+/// </summary>
+public sealed class Late2020VersionPlugin : IVersionPlugin
+{
+    public const string GenerationKey = "Late2020";
+
+    public IReadOnlyCollection<string> VersionKeys { get; } = new[]
+    {
+        // Verified working against this server. The canonical
+        // private-deployment build the codebase has been developed
+        // against since the move to localhost.
+        "december_2020_12_18",
+    };
+
+    public string Generation => GenerationKey;
+
+    public void RegisterStrategies(IServiceCollection services)
+    {
+        // Intentionally empty for now. Generation-specific strategy
+        // services land here as soon as the first controller diverges
+        // (e.g. an IWireFormat keyed on GenerationKey, an IRouteOverride,
+        // etc.). Today every wire shape is implicitly "Late2020" so
+        // there's nothing to differentiate.
+    }
+}