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.

HANDOFF.md

@@ -14,6 +14,11 @@
   the same settings block (one alert box per layout).
 - Tests: `ChromaKeyDetectorTests` + `ChromaKeyApplyTests` (pipe asks ffmpeg to key; offer → accept →
   keyed session feeds the resolver; survives save; auto-apply; alert clip keyed). App build 0 warnings.
+- **TASK 49 retired (creator ruling):** YouTube owns chat moderation — blocked words and hold-for-review
+  run before the API. What we owed it was not a wordlist but not painting what YouTube removed: the chat
+  parser now skips `tombstone` (a deleted message's text is never sent) and `hasDisplayContent:false`.
+  Same commit fixed a live bug: Super Chat read `displayString` instead of `amountDisplayString`, so every
+  amount fell back to a recomposed string.
 
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