Support ack / read in 1-to-1 conversations

Show whether a one-to-one message was received and read, and let trusted contacts see when their messages have been read.

Goal

In a one-to-one conversation, the sender can see that a message was delivered and then read. This gives the same feedback users expect from other messengers, while keeping the read state private from contacts who are not trusted.

Protocols

• XEP-0333: Chat Markers, sending displayed when the reader actually sees a message and handling markable on outgoing messages. Markers are cumulative: one displayed marker covers every earlier message in the conversation.
• XEP-0184: Message Delivery Receipts, optional, for a "delivered" state distinct from "read". Decide whether to support it, or only chat markers.
• XEP-0490: Message Displayed Synchronization, already implemented for syncing our own read position across devices. Outgoing markers must be derived from the same canonical read pointer, never from a second notion of "read".

Scope

• Sending read markers is limited to trusted contacts. Proposed definition: a contact with a mutual presence subscription. Never send to strangers, and never send from a conversation that is not actually being viewed.
• A user setting controls sending: on for trusted contacts (default to decide), or off. When sending is off, displaying the markers other people send is still possible, and the reciprocity rule is a product decision to record.
• Display: a compact per-message state (sent, delivered if XEP-0184 is supported, read) on outgoing messages. Because markers are cumulative, show "read" once on the latest read message rather than on every row.
• Multiple devices: reading on another device (known through XEP-0490) must not send duplicate markers, and a marker already sent is never retracted.
• Encrypted conversations: define whether markers are sent in the clear, as they are metadata only, and document it.

Out of scope

• Group chats and rooms, tracked in a separate issue.
• Typing notifications (XEP-0085).

Dependencies

Build on the consolidated read-state model (#1081) and after the current read-pointer fixes have landed: clearing room unread badges at the live edge, and releasing stale XEP-0490 pending markers. Outgoing markers must follow the forward-only read pointer and never move backwards (#1076). Anchors use MessageRowRef (docs/MESSAGE_IDENTIFIERS.md).

Acceptance

• A marker is sent only when a trusted contact's message is actually displayed with the window focused, and never for a history load or a background catch-up.
• Received markers update the outgoing message state, including when they arrive through MAM after reconnecting.
• The setting is respected immediately and persisted.
• Tests cover: trusted versus untrusted contacts, cumulative markers, markers received out of order, multi-device read with no duplicates, and the setting off.

Start with the consolidated read-state model in #1081 and the current read-pointer fixes, then read docs/MESSAGE_IDENTIFIERS.md for the MessageRowRef anchors. Trace the existing XEP-0490 synchronization and message-state handling before deciding how the listed protocol and trust rules fit. Done means the acceptance tests pass for trusted contacts, cumulative and out-of-order markers, multi-device reads, and the setting off.