docs(events): document ServerAck event and pre-1.0 payload stability

concepts/events.mdx

@@ -144,6 +144,7 @@ pub enum Event {
     Receipt(Receipt),
     UndecryptableMessage(UndecryptableMessage),
     Notification(Arc<OwnedNodeRef>),
+    ServerAck(ServerAck),
 
     // Presence
     ChatPresence(ChatPresenceUpdate),
@@ -210,6 +211,10 @@ pub enum Event {
 The `Event` enum is `#[non_exhaustive]`, so your `match` statements must include a wildcard arm (`_ => {}`). New variants may be added in minor releases without a breaking change.
 </Note>
 
+<Note>
+**Payload stability (pre-1.0):** the payload structs themselves are *not* sealed — while the crate is `0.x`, an existing payload may gain new fields in a minor release, so read the fields you need (e.g. `ack.class`) or keep a `..` rest when destructuring, rather than binding every field. A maybe-absent field is always modeled as `Option<T>`, never an empty-string or zero sentinel. Sealing payloads behind `#[non_exhaustive]` + constructors is deferred to the 1.0 API freeze.
+</Note>
+
 ## Connection Events
 
 ### Connected
@@ -932,6 +937,46 @@ Event::UndecryptableMessage(undec) => {
 }
 ```
 
+### ServerAck
+
+**Emitted:** Observe-only, for every server `<ack>` stanza that carries an id — dispatched independently of the internal send-waiter resolution, so registering a handler never interacts with the send/phash flow.
+
+```rust
+#[derive(Debug, Clone, Serialize)]
+pub struct ServerAck {
+    pub id: String,
+    pub class: Option<String>,
+    pub from: Option<Jid>,
+    pub timestamp: Option<DateTime<Utc>>,
+    pub error: Option<String>,
+}
+```
+
+| Field | Description |
+|-------|-------------|
+| `id` | Id of the acked stanza (for a sent message, its message id). |
+| `class` | Stanza class the ack refers to (`"message"`, `"receipt"`, `"notification"`, `"call"`, …). `None` when the server omits it. |
+| `from` | Chat/entity the ack refers to, when present and parseable. |
+| `timestamp` | Server timestamp from the ack's `t` attribute, when present. For a message ack this is the authoritative send timestamp — the same attribute the [whatsmeow](https://github.com/tulir/whatsmeow) Go library reads into its own `SendResponse.Timestamp`. |
+| `error` | Nack code (e.g. `"479"`), when the ack is actually a nack; `None` for a plain ack. |
+
+<Note>
+Server acks cover every outgoing stanza class, not just messages — filter on `class` rather than correlating ids blind. Dispatch is gated on a registered handler existing for this event kind, so the hot ack path allocates nothing when no consumer subscribes.
+</Note>
+
+**Example:**
+```rust
+Event::ServerAck(ack) => {
+    if ack.class.as_deref() == Some("message") {
+        if let Some(err) = &ack.error {
+            eprintln!("Message {} nacked: {err}", ack.id);
+        } else {
+            println!("Message {} accepted by the server", ack.id);
+        }
+    }
+}
+```
+
 ## Presence Events
 
 ### ChatPresence