feat: cloned rooms get their own copy of the source's save data

Cloning was a shallow copy: the new room's scenes kept pointing at the SOURCE
room's blob until someone saved over them. So every clone of a room shared one
object, a clone read as "already has content" while owning none of it, and the
room a player copied stayed load-bearing for their copy indefinitely. Cloning
RecCenter produced rooms whose data blob was literally room_100_v1.dat, which
is visible in the join log.

Each sub-room's bytes are now copied into the clone's own namespace, keyed by
the CLONE's room id so BlobRouter files them under that room's prefix. Sub-room
0 takes the canonical default name, which matters because the details builder
falls back to exactly that name when a room has no recorded blob — writing the
copy there means both the recorded and the fallback path resolve to real
content. Each copy also gets a RoomDataBlobs row so it is tracked like any
other save.

Two cases deliberately do not clear a reference:

- With no object store configured there is nothing to copy, and a blob name is
  only a reference; clearing it would throw away the player's edits, which is
  strictly worse than the shared reference. That path keeps the old shallow
  behaviour, which is also what the existing user-room clone test asserts.
- A source object that is missing or unreadable leaves the clone empty rather
  than failing the request. The room and its sub-rooms are already committed by
  that point, so a hard failure would strand a half-made room, and an unsaved
  source clones to an empty room anyway.

Everywhere else a reference is either replaced by a real copy or cleared — it
is never left pointing into another room.

DorkNet.Server.Tests/ChatStartConversationTests.cs

@@ -70,6 +70,11 @@ public sealed class ChatStartConversationTests : IClassFixture<DorkNetServerFact
              cannot be opened until someone has already sent a message, which is
              impossible to do first.
              """);
+        Console.WriteLine("=== WITHMEMBERS ===");
+        Console.WriteLine(createBody);
+        Console.WriteLine("=== GET THREAD ===");
+        Console.WriteLine(openBody);
+
         Assert.True(openResponse.IsSuccessStatusCode,
             $"GET /thread/{threadId} -> {(int)openResponse.StatusCode}: {openBody}");
     }