fix(chat): stop painting what YouTube removed, and retire the local word filter

TASK 49 is retired (creator ruling): YouTube owns chat moderation. Its blocked-words filter and
hold-for-review run upstream of `liveChatMessages.list` — held messages are never returned
(https://stackoverflow.com/questions/74511524/) — so a second local list would duplicate a switch
the creator already owns, with a second set of false positives and a second support surface.

What we actually owed the platform, and had wrong:

- The parser added every item to the overlay. A deleted message arrives as a `tombstone` carrying
  no text at all ("it has since been deleted … only liveChatId, type and publishedAt are present" —
  https://developers.google.com/youtube/data/live/reference/rest/v3/liveChatMessages), so every
  deletion rendered as a BLANK chat row. `hasDisplayContent:false` messages and chat-mode events
  did the same.
- Deletion events (`messageDeletedEvent`/`messageRetractedEvent`/`userBannedEvent`) are documented
  but have not been delivered to clients for years (https://stackoverflow.com/questions/44346893/),
  so nothing is built on them; they are skipped if they ever arrive.
- Found while testing: Super Chat read `displayString`, but YouTube's field is `amountDisplayString`
  — every amount silently fell back to a recomposed string, which mangles non-dollar locales. The
  model's own comment said this was the reason the field exists.

Test: `ChatModerationParseTests` — tombstone, no-display-content, chat-mode events all produce no
row; a real Super Chat still parses its pre-rendered amount (constraint 4: reward fields are never
reinterpreted).

Docs: TASK 49 marked retired with the citations, the rule recorded in decisions.md, HANDOFF updated.
verify.sh: 550 passed, 0 failed, 0 warnings.

Services/YouTubeChatService.cs

@@ -106,6 +106,27 @@ public class YouTubeChatService : IDisposable
     /// <summary>One liveChat/messages page → typed chat messages + pagination +
     /// the server's poll cadence. Internal + separated from the HTTP so the six
     /// event parses are deterministically testable without any network.</summary>
+    /// <summary>True for the types that carry something a viewer should read. `snippet.hasDisplayContent`
+    /// is authoritative when present; tombstones and chat-mode events are never shown (they exist to
+    /// mark a deletion or a mode change, and carry no message).</summary>
+    internal static bool HasDisplayableContent(JsonElement snippet, string type)
+    {
+        switch (type)
+        {
+            case "tombstone":
+            case "chatEndedEvent":
+            case "sponsorOnlyModeStartedEvent":
+            case "sponsorOnlyModeEndedEvent":
+            case "messageDeletedEvent":
+            case "messageRetractedEvent":
+            case "userBannedEvent":
+                return false;
+        }
+
+        return !snippet.TryGetProperty("hasDisplayContent", out var has)
+               || has.ValueKind != JsonValueKind.False;
+    }
+
     internal static ChatPage ParsePage(string json)
     {
         var data = JsonSerializer.Deserialize<JsonElement>(json);
@@ -126,9 +147,16 @@ public class YouTubeChatService : IDisposable
         {
             var snippet = item.GetProperty("snippet");
             var type = snippet.TryGetProperty("type", out var t) && t.ValueKind == JsonValueKind.String
-                ? t.GetString()
+                ? t.GetString() ?? "textMessageEvent"
                 : "textMessageEvent";
 
+            // YouTube owns moderation: blocked words, held-for-review messages and deleted
+            // messages are filtered before the feed ever reaches us (2026-11-05 ruling —
+            // TASK 49's local wordlist retired). What DOES arrive are event markers rather
+            // than chat: a `tombstone` (a deleted message's placeholder, carrying no text at
+            // all) and the silent chat-mode events. They get no row on the overlay.
+            if (!HasDisplayableContent(snippet, type)) continue;
+
             var message = new ChatMessage
             {
                 Id = item.TryGetProperty("id", out var idProp) ? idProp.GetString() ?? Guid.NewGuid().ToString() : Guid.NewGuid().ToString(),
@@ -157,9 +185,15 @@ public class YouTubeChatService : IDisposable
                     message.Kind = ChatEventKind.SuperChat;
                     if (snippet.TryGetProperty("superChatDetails", out var scDetails))
                     {
-                        if (scDetails.TryGetProperty("displayString", out var display)
+                        // YouTube names the pre-rendered amount `amountDisplayString`; the old
+                        // `displayString` lookup never matched, so every Super Chat fell back to a
+                        // recomposed amount and mangled non-dollar locales (fixed 2026-11-05).
+                        if (scDetails.TryGetProperty("amountDisplayString", out var display)
                             && display.ValueKind == JsonValueKind.String)
                             message.SuperChatDisplayString = display.GetString();
+                        else if (scDetails.TryGetProperty("displayString", out var legacyDisplay)
+                            && legacyDisplay.ValueKind == JsonValueKind.String)
+                            message.SuperChatDisplayString = legacyDisplay.GetString();
                         if (scDetails.TryGetProperty("amountMicros", out var amt)
                             && amt.ValueKind == JsonValueKind.Number)
                             message.SuperChatAmount = amt.GetDouble() / 1_000_000.0;