release(v1.7.27-alpha): mirror transparency — served-by line + one-click test button

- New "Served by {mirror}" line on the System Update page so operators can see
  which mirror actually served the available manifest (vs. which is configured
  primary). Backend threads the served URL through UpdateState.manifest_mirror.
- New update.test-mirror RPC + per-row lightning-bolt button that pings a
  mirror and renders reachable/latency or error inline under the URL.
- UI polish on the mirrors section: Set Primary, Remove, and the new Test
  action are compact icon buttons; add-mirror form moved into a dialog.
- "What's New" block prepended for v1.7.27-alpha.

21/21 update module tests pass. vue-tsc + vite build clean.

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

core/archipelago/src/api/rpc/dispatcher.rs

@@ -444,6 +444,10 @@ impl RpcHandler {
                 let p = params.unwrap_or(serde_json::json!({}));
                 self.handle_update_set_primary_mirror(&p).await
             }
+            "update.test-mirror" => {
+                let p = params.unwrap_or(serde_json::json!({}));
+                self.handle_update_test_mirror(&p).await
+            }
             "update.apply" => self.handle_update_apply().await,
             "update.git-apply" => self.handle_update_git_apply().await,
             "update.rollback" => self.handle_update_rollback().await,

core/archipelago/src/api/rpc/update.rs

@@ -40,6 +40,7 @@ impl RpcHandler {
             "last_check": state.last_check,
             "update_available": update_info.is_some(),
             "update": update_info,
+            "manifest_mirror": state.manifest_mirror,
         }))
     }
 
@@ -195,6 +196,7 @@ impl RpcHandler {
             "update_available": state.available_update.is_some(),
             "update_in_progress": state.update_in_progress,
             "rollback_available": state.rollback_available,
+            "manifest_mirror": state.manifest_mirror,
             "download_progress": if active || completed {
                 Some(serde_json::json!({
                     "bytes_downloaded": downloaded,
@@ -290,6 +292,20 @@ impl RpcHandler {
         Ok(serde_json::json!({ "mirrors": list }))
     }
 
+    /// Ping a mirror's manifest URL. Returns reachability, wall-clock
+    /// latency, and HTTP status. Params: `{ url }`.
+    pub(super) async fn handle_update_test_mirror(
+        &self,
+        params: &serde_json::Value,
+    ) -> Result<serde_json::Value> {
+        let url = params
+            .get("url")
+            .and_then(|v| v.as_str())
+            .ok_or_else(|| anyhow::anyhow!("missing url"))?;
+        let result = update::test_mirror(url).await;
+        Ok(serde_json::to_value(result)?)
+    }
+
     /// Move a mirror to the top of the list so it's tried first.
     /// Params: `{ url }`.
     pub(super) async fn handle_update_set_primary_mirror(