feat(mcp+lifecycle): MCP degraded-startup reporting, lane event schema, lane completion hardening

Add MCP structured degraded-startup classification (P2.10):
- classify MCP failures as startup/handshake/config/partial
- expose failed_servers + recovery_recommendations in tool output
- add mcp_degraded output field with server_name, failure_mode, recoverable

Canonical lane event schema (P2.7):
- add LaneEventName variants for all lifecycle states
- wire LaneEvent::new with full 3-arg signature (event, status, emitted_at)
- emit typed events for Started, Blocked, Failed, Finished

Fix let mut executor for search test binary
Fix lane_completion unused import warnings

Note: mcp_stdio::manager_discovery_report test has pre-existing failure on clean main, unrelated to this commit.

rust/crates/runtime/src/lane_events.rs

@@ -0,0 +1,241 @@
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum LaneEventName {
+    #[serde(rename = "lane.started")]
+    Started,
+    #[serde(rename = "lane.ready")]
+    Ready,
+    #[serde(rename = "lane.prompt_misdelivery")]
+    PromptMisdelivery,
+    #[serde(rename = "lane.blocked")]
+    Blocked,
+    #[serde(rename = "lane.red")]
+    Red,
+    #[serde(rename = "lane.green")]
+    Green,
+    #[serde(rename = "lane.commit.created")]
+    CommitCreated,
+    #[serde(rename = "lane.pr.opened")]
+    PrOpened,
+    #[serde(rename = "lane.merge.ready")]
+    MergeReady,
+    #[serde(rename = "lane.finished")]
+    Finished,
+    #[serde(rename = "lane.failed")]
+    Failed,
+    #[serde(rename = "lane.reconciled")]
+    Reconciled,
+    #[serde(rename = "lane.merged")]
+    Merged,
+    #[serde(rename = "lane.superseded")]
+    Superseded,
+    #[serde(rename = "lane.closed")]
+    Closed,
+    #[serde(rename = "branch.stale_against_main")]
+    BranchStaleAgainstMain,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum LaneEventStatus {
+    Running,
+    Ready,
+    Blocked,
+    Red,
+    Green,
+    Completed,
+    Failed,
+    Reconciled,
+    Merged,
+    Superseded,
+    Closed,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum LaneFailureClass {
+    PromptDelivery,
+    TrustGate,
+    BranchDivergence,
+    Compile,
+    Test,
+    PluginStartup,
+    McpStartup,
+    McpHandshake,
+    GatewayRouting,
+    ToolRuntime,
+    Infra,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct LaneEventBlocker {
+    #[serde(rename = "failureClass")]
+    pub failure_class: LaneFailureClass,
+    pub detail: String,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct LaneEvent {
+    pub event: LaneEventName,
+    pub status: LaneEventStatus,
+    #[serde(rename = "emittedAt")]
+    pub emitted_at: String,
+    #[serde(rename = "failureClass", skip_serializing_if = "Option::is_none")]
+    pub failure_class: Option<LaneFailureClass>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub detail: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub data: Option<Value>,
+}
+
+impl LaneEvent {
+    #[must_use]
+    pub fn new(
+        event: LaneEventName,
+        status: LaneEventStatus,
+        emitted_at: impl Into<String>,
+    ) -> Self {
+        Self {
+            event,
+            status,
+            emitted_at: emitted_at.into(),
+            failure_class: None,
+            detail: None,
+            data: None,
+        }
+    }
+
+    #[must_use]
+    pub fn started(emitted_at: impl Into<String>) -> Self {
+        Self::new(LaneEventName::Started, LaneEventStatus::Running, emitted_at)
+    }
+
+    #[must_use]
+    pub fn finished(emitted_at: impl Into<String>, detail: Option<String>) -> Self {
+        Self::new(LaneEventName::Finished, LaneEventStatus::Completed, emitted_at)
+            .with_optional_detail(detail)
+    }
+
+    #[must_use]
+    pub fn blocked(emitted_at: impl Into<String>, blocker: &LaneEventBlocker) -> Self {
+        Self::new(LaneEventName::Blocked, LaneEventStatus::Blocked, emitted_at)
+            .with_failure_class(blocker.failure_class)
+            .with_detail(blocker.detail.clone())
+    }
+
+    #[must_use]
+    pub fn failed(emitted_at: impl Into<String>, blocker: &LaneEventBlocker) -> Self {
+        Self::new(LaneEventName::Failed, LaneEventStatus::Failed, emitted_at)
+            .with_failure_class(blocker.failure_class)
+            .with_detail(blocker.detail.clone())
+    }
+
+    #[must_use]
+    pub fn with_failure_class(mut self, failure_class: LaneFailureClass) -> Self {
+        self.failure_class = Some(failure_class);
+        self
+    }
+
+    #[must_use]
+    pub fn with_detail(mut self, detail: impl Into<String>) -> Self {
+        self.detail = Some(detail.into());
+        self
+    }
+
+    #[must_use]
+    pub fn with_optional_detail(mut self, detail: Option<String>) -> Self {
+        self.detail = detail;
+        self
+    }
+
+    #[must_use]
+    pub fn with_data(mut self, data: Value) -> Self {
+        self.data = Some(data);
+        self
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+
+    use super::{
+        LaneEvent, LaneEventBlocker, LaneEventName, LaneEventStatus, LaneFailureClass,
+    };
+
+    #[test]
+    fn canonical_lane_event_names_serialize_to_expected_wire_values() {
+        let cases = [
+            (LaneEventName::Started, "lane.started"),
+            (LaneEventName::Ready, "lane.ready"),
+            (
+                LaneEventName::PromptMisdelivery,
+                "lane.prompt_misdelivery",
+            ),
+            (LaneEventName::Blocked, "lane.blocked"),
+            (LaneEventName::Red, "lane.red"),
+            (LaneEventName::Green, "lane.green"),
+            (LaneEventName::CommitCreated, "lane.commit.created"),
+            (LaneEventName::PrOpened, "lane.pr.opened"),
+            (LaneEventName::MergeReady, "lane.merge.ready"),
+            (LaneEventName::Finished, "lane.finished"),
+            (LaneEventName::Failed, "lane.failed"),
+            (LaneEventName::Reconciled, "lane.reconciled"),
+            (LaneEventName::Merged, "lane.merged"),
+            (LaneEventName::Superseded, "lane.superseded"),
+            (LaneEventName::Closed, "lane.closed"),
+            (
+                LaneEventName::BranchStaleAgainstMain,
+                "branch.stale_against_main",
+            ),
+        ];
+
+        for (event, expected) in cases {
+            assert_eq!(serde_json::to_value(event).expect("serialize event"), json!(expected));
+        }
+    }
+
+    #[test]
+    fn failure_classes_cover_canonical_taxonomy_wire_values() {
+        let cases = [
+            (LaneFailureClass::PromptDelivery, "prompt_delivery"),
+            (LaneFailureClass::TrustGate, "trust_gate"),
+            (LaneFailureClass::BranchDivergence, "branch_divergence"),
+            (LaneFailureClass::Compile, "compile"),
+            (LaneFailureClass::Test, "test"),
+            (LaneFailureClass::PluginStartup, "plugin_startup"),
+            (LaneFailureClass::McpStartup, "mcp_startup"),
+            (LaneFailureClass::McpHandshake, "mcp_handshake"),
+            (LaneFailureClass::GatewayRouting, "gateway_routing"),
+            (LaneFailureClass::ToolRuntime, "tool_runtime"),
+            (LaneFailureClass::Infra, "infra"),
+        ];
+
+        for (failure_class, expected) in cases {
+            assert_eq!(
+                serde_json::to_value(failure_class).expect("serialize failure class"),
+                json!(expected)
+            );
+        }
+    }
+
+    #[test]
+    fn blocked_and_failed_events_reuse_blocker_details() {
+        let blocker = LaneEventBlocker {
+            failure_class: LaneFailureClass::McpStartup,
+            detail: "broken server".to_string(),
+        };
+
+        let blocked = LaneEvent::blocked("2026-04-04T00:00:00Z", &blocker);
+        let failed = LaneEvent::failed("2026-04-04T00:00:01Z", &blocker);
+
+        assert_eq!(blocked.event, LaneEventName::Blocked);
+        assert_eq!(blocked.status, LaneEventStatus::Blocked);
+        assert_eq!(blocked.failure_class, Some(LaneFailureClass::McpStartup));
+        assert_eq!(failed.event, LaneEventName::Failed);
+        assert_eq!(failed.status, LaneEventStatus::Failed);
+        assert_eq!(failed.detail.as_deref(), Some("broken server"));
+    }
+}