add IMAP write operations, special folder role detection

Extend IMAPClientProtocol with storeFlags, moveMessage, copyMessage,
expunge, appendMessage, capabilities methods. Implement all six in
IMAPClient actor using NIOIMAPCore typed commands. Add multi-part
command support to IMAPConnection/IMAPCommandRunner for APPEND.
MockIMAPClient tracks all write calls for testing. SyncCoordinator
detects mailbox roles from LIST attributes with name-based fallback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Packages/MagnumOpusCore/Sources/IMAPClient/IMAPConnection.swift

@@ -46,6 +46,19 @@ actor IMAPConnection {
 		}
 	}
 
+	/// Send multiple command parts as a single multi-part command (e.g. APPEND).
+	/// The tag must match the tag embedded in the first part.
+	func sendMultiPartCommand(_ tag: String, parts: [CommandStreamPart]) async throws -> [Response] {
+		guard let channel else { throw IMAPError.notConnected }
+		let handler = responseHandler
+		return try await withCheckedThrowingContinuation { continuation in
+			handler.sendCommand(tag: tag, continuation: continuation)
+			for part in parts {
+				channel.writeAndFlush(IMAPClientHandler.Message.part(part), promise: nil)
+			}
+		}
+	}
+
 	func disconnect() async throws {
 		try await channel?.close()
 		channel = nil