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/Services/RoomCodeService.cs

@@ -0,0 +1,21 @@
+namespace DorkNet.Server.Services;
+
+public static class RoomCodeService
+{
+    private const string Alphabet = "23456789ABCDEFGHJKLMNPQRSTUVWXYZ";
+
+    public static string Normalize(string? code)
+        => (code ?? string.Empty).Trim().Replace("-", string.Empty).ToUpperInvariant();
+
+    public static string Generate(long instanceId)
+    {
+        var value = (ulong)HashCode.Combine(instanceId, 0x52_52_4d_43);
+        Span<char> chars = stackalloc char[6];
+        for (var i = 0; i < chars.Length; i++)
+        {
+            chars[i] = Alphabet[(int)(value % (uint)Alphabet.Length)];
+            value /= (uint)Alphabet.Length;
+        }
+        return new string(chars);
+    }
+}