feat: add inbound username support and USync username protocol

Aligns with Baileys upstream PR #2480. WhatsApp is rolling out an
inbound username field that maps to the user's LID. This change is
purely additive — it captures and propagates the new optional field
through types, decoders, USync queries, and group/contact events.

Skipped intentionally to preserve InfiniteAPI's LID/PN customization:
- handleGroupNotification in messages-recv.ts (custom LID->PN flow on
  groups.upsert / participants.map). Username for these specific
  events still flows via process-message's emitParticipantsUpdate
  (reads message.key.participantUsername added in decode-wa-message).

Carousel/buttons code (messages.ts, messages-send.ts) untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Utils/decode-wa-message.ts

@@ -242,10 +242,14 @@ export function decodeMessageNode(stanza: BinaryNode, meId: string, meLid: strin
 	const key: WAMessageKey = {
 		remoteJid: chatId,
 		remoteJidAlt: !isJidGroup(chatId) ? addressingContext.senderAlt : undefined,
+		remoteJidUsername: !isJidGroup(chatId)
+			? stanza.attrs.peer_recipient_username || stanza.attrs.recipient_username
+			: undefined,
 		fromMe,
 		id: msgId,
 		participant,
 		participantAlt: isJidGroup(chatId) ? addressingContext.senderAlt : undefined,
+		participantUsername: stanza.attrs.participant ? stanza.attrs.participant_username : undefined,
 		addressingMode: addressingContext.addressingMode,
 		...(msgType === 'newsletter' && stanza.attrs.server_id ? { server_id: stanza.attrs.server_id } : {})
 	}