fix(check): merge anonymous resignup viewer into Steam-linked viewer

GameStart already detects the Steam-vs-UDID mismatch produced by
wipe-and-resignup; it now also reclaims the orphan. New
ViewerRepository.MergeAnonymousViewerInto transfers the fresh UDID
from V_new onto V_old in one save (freeing the unique-index slot),
then deletes V_new in a second save. Partial-failure mode is a
benign null-UDID viewer; two rows never contend for the same UDID.
Side benefit: future GetViewerByUdid lookups now short-circuit to
V_old without going through the Steam handler.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Controllers/CheckController.cs

@@ -54,6 +54,10 @@ public class CheckController : SVSimController
             if (udidViewer is not null && udidViewer.Id != fullViewer.Id)
             {
                 rewriteViewerId = fullViewer.Id;
+                // Reclaim the orphan: transfer the fresh UDID onto the Steam-resolved viewer
+                // and delete the just-created blank anonymous one. Future GetViewerByUdid
+                // calls then short-circuit to V_old without going through the Steam handler.
+                await _viewerRepository.MergeAnonymousViewerInto(udidViewer.Id, fullViewer.Id);
             }
         }