feat(guild_chat): post text + stamp

Implements PostTextOrStampAsync: validates membership (NotInGuild),
validates stamp id against UsableStampList (PermissionDenied), rejects
type other than 0 or 1, allocates next per-guild message_id, appends row.
Wires /guild_chat/post controller action mapping failures to result_code=2.
Response is GuildChatPostResponse (empty body per spec). 7 integration tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Services/Guild/GuildChatService.cs

@@ -65,10 +65,38 @@ public sealed class GuildChatService : IGuildChatService
         return new(msgs, wait);
     }
 
-    // ── Not-yet-implemented stubs ────────────────────────────────────────────
+    // ── Post text / stamp ────────────────────────────────────────────────────
 
-    public Task<ChatPostResult> PostTextOrStampAsync(long viewerId, int type, string message, CancellationToken ct = default)
-        => throw new NotImplementedException();
+    public async Task<ChatPostResult> PostTextOrStampAsync(long viewerId, int type, string message, CancellationToken ct = default)
+    {
+        var m = await _members.GetMembershipAsync(viewerId, ct);
+        if (m is null) return new(false, null);
+
+        var cfg = _cfg.Get<GuildConfig>();
+        if (type == 1)
+        {
+            if (!int.TryParse(message, out var stampId) || !cfg.UsableStampList.Contains(stampId))
+                return new(false, null);
+        }
+        else if (type != 0)
+        {
+            // Only NORMAL (0) and STAMP (1) are valid via this endpoint.
+            return new(false, null);
+        }
+
+        var now    = DateTime.UtcNow;
+        var nextId = await _msgs.GetMaxMessageIdAsync(m.GuildId, ct) + 1;
+        var stored = await _msgs.AppendAsync(new GuildChatMessage
+        {
+            GuildId        = m.GuildId,
+            MessageId      = nextId,
+            AuthorViewerId = viewerId,
+            MessageType    = type == 1 ? GuildChatMessageType.Stamp : GuildChatMessageType.Normal,
+            Body           = message,
+            CreatedAt      = now,
+        }, ct);
+        return new(true, stored.MessageId);
+    }
 
     public Task<ChatPostResult> PostDeckAsync(long viewerId, string deckPayloadJson, CancellationToken ct = default)
         => throw new NotImplementedException();