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/update.rs

@@ -218,6 +218,11 @@ pub struct UpdateState {
     pub rollback_available: bool,
     #[serde(default)]
     pub schedule: UpdateSchedule,
+    /// URL of the mirror whose manifest populated `available_update`.
+    /// Surfaces in the UI so operators can tell at a glance which mirror
+    /// their node actually hit (vs. just which is configured primary).
+    #[serde(default)]
+    pub manifest_mirror: Option<String>,
 }
 
 impl Default for UpdateState {
@@ -229,6 +234,7 @@ impl Default for UpdateState {
             update_in_progress: false,
             rollback_available: false,
             schedule: UpdateSchedule::DailyCheck,
+            manifest_mirror: None,
         }
     }
 }
@@ -260,6 +266,7 @@ pub async fn load_state(data_dir: &Path) -> Result<UpdateState> {
         // it unconditionally; the next check_for_updates will repopulate
         // if there's genuinely something newer.
         state.available_update = None;
+        state.manifest_mirror = None;
         save_state(data_dir, &state).await?;
     }
     Ok(state)
@@ -328,6 +335,7 @@ pub async fn check_for_updates(data_dir: &Path) -> Result<UpdateState> {
                                 "Update available"
                             );
                             state.available_update = Some(manifest);
+                            state.manifest_mirror = Some(manifest_url.clone());
                         } else {
                             // Manifest version matches us or is behind
                             // us — either we're current, or this mirror
@@ -347,6 +355,7 @@ pub async fn check_for_updates(data_dir: &Path) -> Result<UpdateState> {
                                 // break: another mirror could be ahead.
                                 continue 'mirrors;
                             }
+                            state.manifest_mirror = None;
                             state.available_update = None;
                         }
                         handled = true;
@@ -375,6 +384,66 @@ pub async fn check_for_updates(data_dir: &Path) -> Result<UpdateState> {
     Ok(state)
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct MirrorTestResult {
+    pub reachable: bool,
+    pub latency_ms: u64,
+    pub http_status: Option<u16>,
+    pub error: Option<String>,
+}
+
+/// Ping a mirror's manifest URL and return reachability + wall-clock
+/// latency. Used by the "Test mirror" button so operators can sanity-
+/// check a newly added mirror without running a full update check.
+pub async fn test_mirror(url: &str) -> MirrorTestResult {
+    let client = match reqwest::Client::builder()
+        .timeout(std::time::Duration::from_secs(10))
+        .connect_timeout(std::time::Duration::from_secs(5))
+        .build()
+    {
+        Ok(c) => c,
+        Err(e) => {
+            return MirrorTestResult {
+                reachable: false,
+                latency_ms: 0,
+                http_status: None,
+                error: Some(format!("client build failed: {}", e)),
+            }
+        }
+    };
+    let start = std::time::Instant::now();
+    match client.get(url).send().await {
+        Ok(resp) => {
+            let latency_ms = start.elapsed().as_millis() as u64;
+            let status = resp.status();
+            if status.is_success() {
+                MirrorTestResult {
+                    reachable: true,
+                    latency_ms,
+                    http_status: Some(status.as_u16()),
+                    error: None,
+                }
+            } else {
+                MirrorTestResult {
+                    reachable: false,
+                    latency_ms,
+                    http_status: Some(status.as_u16()),
+                    error: Some(format!("HTTP {}", status.as_u16())),
+                }
+            }
+        }
+        Err(e) => {
+            let latency_ms = start.elapsed().as_millis() as u64;
+            MirrorTestResult {
+                reachable: false,
+                latency_ms,
+                http_status: None,
+                error: Some(e.to_string()),
+            }
+        }
+    }
+}
+
 /// Get current update status without checking remote.
 pub async fn get_status(data_dir: &Path) -> Result<UpdateState> {
     load_state(data_dir).await
@@ -1155,6 +1224,7 @@ mod tests {
             update_in_progress: false,
             rollback_available: true,
             schedule: UpdateSchedule::AutoApply,
+            manifest_mirror: None,
         };
         let json = serde_json::to_string(&state).unwrap();
         let deserialized: UpdateState = serde_json::from_str(&json).unwrap();
@@ -1265,6 +1335,10 @@ mod tests {
             update_in_progress: true,
             rollback_available: false,
             schedule: UpdateSchedule::Manual,
+            manifest_mirror: Some(
+                "https://git.tx1138.com/lfg2025/archy/raw/branch/main/releases/manifest.json"
+                    .to_string(),
+            ),
         };
         save_state(dir.path(), &state).await.unwrap();
         let loaded = load_state(dir.path()).await.unwrap();