Implement textDate autoformatter for InstantPage V2 rich bubbles

Render RichText.textDate (API constructor textDate(flags, text, date)) as a
locale/timezone-correct, relative-aware, tappable date inside V2 rich-message
bubbles, reusing the existing stringForEntityFormattedDate autoformatter.

- Model: add RichText.textDate(text📅format:) with Postbox coding,
  FlatBuffers schema (RichText.fbs; flatc regenerates), Equatable and plainText;
  parse flags->DateTimeFormat? at the API boundary (flags==0 => nil, matching
  the messageEntityFormattedDate convention) with a symmetric apiRichText()
  round-trip. The new case is handled in every exhaustive RichText switch
  (SyncCore, ApiUtils, InstantPageTextItem, InstantPageAnchorPath,
  InstantPagePreviewText, BrowserReadability, BrowserMarkdown).
- Render: attributedStringForRichText gains an optional formatDate closure;
  the V2 layout builds it from the message's strings/dateTimeFormat and threads
  it to every text-building call site, formatting the timestamp and tagging the
  run with TelegramTextAttributes.Date.
- Tappable: the Date attribute is added to linkSelectionRects hit-testing and
  mapped in the rich bubble's entityTapContent to the existing .date tap action
  (opens the date context menu).
- Live update: the V2 layout accumulates a page-wide formattedDateUpdatePeriod
  (>=10s) for relative dates; the rich bubble schedules a refresh timer on it,
  recreating it only when the period changes. The currentPageLayout cache is
  bypassed for relative-date pages so the timer-driven relayout actually
  re-formats the date.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

submodules/TelegramStringFormatting/Sources/InstantPagePreviewText.swift

@@ -46,7 +46,7 @@ extension RichText {
             return "Fx"
         case let .textCustomEmoji(_, alt):
             return alt
-        case let .textAutoEmail(value), let .textAutoPhone(value), let .textAutoUrl(value), let .textBankCard(value), let .textBotCommand(value), let .textCashtag(value), let .textHashtag(value), let .textMention(value), let .textMentionName(value, _), let .textSpoiler(value):
+        case let .textAutoEmail(value), let .textAutoPhone(value), let .textAutoUrl(value), let .textBankCard(value), let .textBotCommand(value), let .textCashtag(value), let .textHashtag(value), let .textMention(value), let .textMentionName(value, _), let .textSpoiler(value), let .textDate(value, _, _):
             return value.previewText()
         }
     }