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.

TASKS/task-49-chat-profanity-filter.md

@@ -4,7 +4,30 @@
 > **Research: [`TASKS/research-store-certification.md`](research-store-certification.md) §12.**
 > Carved out 2026-09-27; shares a scope lock with nothing else.
 
-**Status:** ☐ Queued — not blocked (does not depend on the route decision)
+**Status:** ❌ **Retired 2026-11-05** — YouTube owns chat moderation (creator ruling). What replaced it:
+`Services/YouTubeChatService.cs` no longer turns YouTube's own moderation markers into chat rows
+(`tombstone`, `hasDisplayContent:false`, chat-mode events), with tests in
+`ytLive.Tests/ChatModerationParseTests.cs`. The reasoning and citations are below and in
+`docs/ai/decisions.md`; the body of this file is kept so the option is not re-derived from scratch.
+
+### Why retired
+
+- YouTube's **blocked words** filter ("block live chat messages that contain or closely match certain
+  words") and **hold for review** both run upstream of the API: blocked/held messages are not returned
+  by `liveChatMessages.list` (community evidence: held messages never appear —
+  <https://stackoverflow.com/questions/74511524/>). Ours would be a second list, a second set of false
+  positives and a second support surface for a switch the creator already owns.
+- Deleted messages arrive as a `tombstone` — "signifies that a message used to exist … it has since
+  been deleted … only liveChatId, type and publishedAt are present"
+  (<https://developers.google.com/youtube/data/live/reference/rest/v3/liveChatMessages>). The parser had
+  no filter, so a deletion rendered as a **blank chat row**. That is the actual defect, and it is fixed.
+- Deletion events (`messageDeletedEvent`/`messageRetractedEvent`/`userBannedEvent`) are documented in
+  the resource but have not been delivered to clients for years
+  (<https://stackoverflow.com/questions/44346893/>), so no client-side moderation machinery can be built
+  on them. The parser skips them if they ever do arrive.
+
+The constraints below are therefore **moot for us and binding on YouTube** — except constraint 4, which is
+why the Super Chat parse path still asserts that reward fields are never reinterpreted.
 
 **Goal:** let the creator decide what reaches the projected chat box, without the app ever
 becoming a moderator that stores what people said.