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/Compat/ClientVersion.cs

@@ -0,0 +1,28 @@
+namespace DorkNet.Server.Compat;
+
+/// <summary>Identifies a specific Rec Room client build.
+/// <para>
+/// <see cref="Key"/> is the wire-format identifier the client mod
+/// stamps on every request via the <c>X-DorkNet-Version</c> header.
+/// Format is <c>{branch}_{semver}</c>: <c>march_2020_03_10</c>,
+/// <c>december_2020_12_18</c>. Keep it lowercase, underscore-separated;
+/// it shows up in admin logs and error responses.
+/// </para>
+/// <para>
+/// <see cref="Generation"/> is the wire-format-compatibility group:
+/// every version inside the same generation shares the same wire
+/// shapes (JSON DTO casing, route names, etc.) and therefore the same
+/// strategy implementations. Adding a new build that's protocol-compatible
+/// with an existing generation = a one-line registration; only a new
+/// generation entails new strategy code.
+/// </para>
+/// </summary>
+public sealed record ClientVersion(string Key, string Generation)
+{
+    /// <summary>Sentinel returned when a request has no
+    /// <c>X-DorkNet-Version</c> header AND no
+    /// <c>DorkNet:DefaultClientVersion</c> is configured. Controllers
+    /// can branch on this to either fall through to a sensible default
+    /// or 426 the request.</summary>
+    public static readonly ClientVersion Unknown = new("unknown", "unknown");
+}