Stop BRouter contention kills under rapid editing

When a user rapidly moves waypoints in the Planner, BRouter's
thread-priority watchdog kills older in-flight requests to free a thread
for newer ones, surfacing to the user as "operation killed by
thread-priority-watchdog" errors.

Two fixes:

1. Bump the BRouter thread pool on the dedicated host from 4 to 16.
   `BROUTER_THREADS` is now a Dockerfile ENV (default 4 for
   flagship/CI/dev) overridden to 16 in `infrastructure/brouter-host/`.
   The host has 32 GB of RAM and idle cores; 4 threads was a flagship-era
   constraint that no longer applies.

2. Cancel the previous in-flight `/api/route` request when a new one
   starts. `use-routing.ts` now keeps an `AbortController` in a ref,
   aborts it at the top of each `computeRoute`, and bails quietly on
   `AbortError` so the newer call drives state.

Together these eliminate both the "real" contention (at the BRouter
level) and the wasted work (BRouter computing a route the client no
longer cares about).

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

apps/planner/app/lib/use-routing.ts

@@ -51,6 +51,10 @@ export function useRouting(yjs: YjsState | null, sessionId: string) {
   const debounceTimer = useRef<ReturnType<typeof setTimeout>>(undefined);
   const lastGoodWaypointsRef = useRef<WaypointData[] | null>(null);
   const restoringRef = useRef(false);
+  // Cancels the in-flight /api/route call when a newer one starts. Without
+  // this, rapid edits pile up on BRouter's thread pool and older requests
+  // get killed by its contention watchdog, surfacing as spurious errors.
+  const inflightAbortRef = useRef<AbortController | null>(null);
 
   // Host election via Yjs awareness
   useEffect(() => {
@@ -85,6 +89,9 @@ export function useRouting(yjs: YjsState | null, sessionId: string) {
       const snapshotBeforeCompute = getWaypointsFromYjs(yjs.waypoints);
 
       setComputing(true);
+      inflightAbortRef.current?.abort();
+      const controller = new AbortController();
+      inflightAbortRef.current = controller;
       try {
         const response = await fetch("/api/route", {
           method: "POST",
@@ -95,6 +102,7 @@ export function useRouting(yjs: YjsState | null, sessionId: string) {
             noGoAreas: noGoAreas.length > 0 ? noGoAreas : undefined,
             sessionId,
           }),
+          signal: controller.signal,
         });
 
         if (response.status === 429) {
@@ -146,12 +154,18 @@ export function useRouting(yjs: YjsState | null, sessionId: string) {
             yjs.routeData.set("bikeroutes", JSON.stringify(enriched.bikeroutes));
           }
         });
-      } catch {
+      } catch (err) {
+        // A superseding request aborted this one — leave state alone so
+        // the newer call's result becomes authoritative.
+        if ((err as Error)?.name === "AbortError") return;
         setRouteError("failed");
         restoreWaypoints(yjs, lastGoodWaypointsRef.current ?? snapshotBeforeCompute, restoringRef);
       } finally {
+        if (inflightAbortRef.current === controller) {
+          inflightAbortRef.current = null;
           setComputing(false);
         }
+      }
     },
     [yjs, isHost],
   );

docker/brouter/Dockerfile

@@ -38,9 +38,16 @@ EXPOSE 17777
 # The default keeps the flagship's single-instance footprint small.
 ENV JAVA_OPTS="-Xmx1024M -Xms256M -Xmn64M"
 
+# Routing thread pool. When saturated, BRouter's thread-priority watchdog
+# kills the oldest in-flight request ("contention! ms killed …") to free a
+# thread for the new one — the caller then sees a cancelled response. Bump
+# this on hosts with spare cores/RAM to absorb burst traffic. Planet host
+# sets it to 16; flagship/CI keep the default.
+ENV BROUTER_THREADS=4
+
 # BRouter server: <segmentdir> <profiledir> <customprofiledir> <port> <maxthreads>
-# Shell form so $JAVA_OPTS expands at container start.
+# Shell form so $JAVA_OPTS and $BROUTER_THREADS expand at container start.
 CMD java $JAVA_OPTS -DmaxRunningTime=300 -cp brouter.jar \
     btools.server.RouteServer \
     /data/segments /data/profiles /data/profiles \
-    17777 4
+    17777 $BROUTER_THREADS

infrastructure/brouter-host/docker-compose.yml

@@ -19,8 +19,12 @@ services:
     # Planet-scale coverage: segments live on the host and are mounted in.
     # 8 GB heap for segment cache; -Xms generous because routing is
     # memory-heavy and we don't benefit from a slow JVM warmup.
+    # 16 routing threads on the 32 GB / multi-core dedicated host absorbs
+    # burst traffic from "click around wildly" editing without triggering
+    # BRouter's contention watchdog on the 4-thread default.
     environment:
       JAVA_OPTS: "-Xmx8g -Xms512M"
+      BROUTER_THREADS: "16"
     volumes:
       - ./segments:/data/segments:ro
     networks: