fix(signup): prestore client SID→UDID mapping so game_start can decrypt

After /tool/signup the client switches to SID-only headers (no UDID), so
the next request's body can't be decrypted unless the server already
knows the SID's UDID. ShadowverseSessionService now mirrors the client's
Cute/Cryptographer.MakeMd5(viewerId + udid) formula (salt
"r!I@ws8e5i="), and ToolController.Signup prestores the mapping at the
end. Verified against a live signup capture: viewerId=1 +
udid=62747917-93bc-454c-abb4-ef423b3c9317 produces the captured SID
dc4aac79d35fe15dfb6262e0071bb03c.

Note: this only fixes the fresh-signup path. Clients restarting with a
cached viewer_id (which skip /tool/signup entirely) still hit the same
issue — separate follow-up.

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

SVSim.UnitTests/Services/ShadowverseSessionServiceTests.cs

@@ -0,0 +1,38 @@
+using NUnit.Framework;
+using SVSim.EmulatedEntrypoint.Services;
+
+namespace SVSim.UnitTests.Services;
+
+public class ShadowverseSessionServiceTests
+{
+    /// <summary>
+    /// Fixture captured live from a fresh signup against this server. The client computed this
+    /// exact SID locally and sent it on the next /check/game_start request. Pinning the formula
+    /// here means any future refactor of <see cref="ShadowverseSessionService.ComputeClientSessionId"/>
+    /// that drifts from <c>Cute/Cryptographer.MakeMd5(viewerId + udid)</c> will fail this test
+    /// before the user discovers it as a decrypt failure on game_start.
+    /// </summary>
+    [Test]
+    public void ComputeClientSessionId_matches_captured_fixture()
+    {
+        var svc = new ShadowverseSessionService();
+        const long viewerId = 1;
+        var udid = new System.Guid("62747917-93bc-454c-abb4-ef423b3c9317");
+
+        string sid = svc.ComputeClientSessionId(viewerId, udid);
+
+        Assert.That(sid, Is.EqualTo("dc4aac79d35fe15dfb6262e0071bb03c"));
+    }
+
+    [Test]
+    public void StoreSessionForViewer_makes_sid_resolvable_to_udid()
+    {
+        var svc = new ShadowverseSessionService();
+        const long viewerId = 1;
+        var udid = new System.Guid("62747917-93bc-454c-abb4-ef423b3c9317");
+
+        svc.StoreSessionForViewer(viewerId, udid);
+
+        Assert.That(svc.GetUdidFromSessionId("dc4aac79d35fe15dfb6262e0071bb03c"), Is.EqualTo(udid));
+    }
+}