feat(guild): /guild/create + populated /guild/info

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

SVSim.Database/Services/Guild/GuildIdGenerator.cs

@@ -0,0 +1,26 @@
+namespace SVSim.Database.Services.Guild;
+
+public interface IGuildIdGenerator
+{
+    Task<int> NextAsync(Func<int, CancellationToken, Task<bool>> existsAsync, CancellationToken ct);
+}
+
+public sealed class GuildIdGenerator : IGuildIdGenerator
+{
+    private static readonly System.Security.Cryptography.RandomNumberGenerator _rng =
+        System.Security.Cryptography.RandomNumberGenerator.Create();
+
+    public async Task<int> NextAsync(Func<int, CancellationToken, Task<bool>> existsAsync, CancellationToken ct)
+    {
+        const int min = 100_000_000, max = 999_999_999;
+        const int attempts = 16;
+        for (int i = 0; i < attempts; i++)
+        {
+            var bytes = new byte[4]; _rng.GetBytes(bytes);
+            var raw = BitConverter.ToUInt32(bytes, 0);
+            int id = min + (int)(raw % (uint)(max - min + 1));
+            if (!await existsAsync(id, ct)) return id;
+        }
+        throw new InvalidOperationException("Exhausted guild_id collision retries — id space too saturated.");
+    }
+}