feat(component-failover-adoption): trigger apiserver's rejoin from the idle-period heartbeat path

xcecli_heartbeat() is XCECLI_FF_CALL-wrapped -- a deliberately
non-retrying fire-and-forget path that does its own best-effort
one-shot reconnect but never calls xcecli_invoke_rejoin() (that only
fires from the write-call retry path). Left as-is, a controller
failover with no in-flight write request would silently repair the
transport connection without ever re-joining, permanently starving
push notifications.

runHeartbeat() gains two callback hooks fired at the right points --
onFailureToSuccess on a genuine transition, onSuccessTick on every
successful tick thereafter -- wired directly to task 4.3's existing
setNeedsRejoin()/checkAndPerformRejoin() from StartHeartbeat(). No
second rejoin mechanism: checkAndPerformRejoin() is a cheap no-op when
nothing is pending, so calling it unconditionally on every successful
tick is safe and matches the requirement that retry isn't gated on the
transition edge alone.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NNB4zGPCsHyDUCDJyMbiQN

src/frontend/apiserver/backend/clib.go

@@ -349,10 +349,17 @@ func Cleanup() {
 
 // StartHeartbeat launches a goroutine that sends periodic xcecli_heartbeat
 // keep-alive messages to the controller. Stops when ctx is cancelled.
+//
+// Wires runHeartbeat's failure-to-success and every-successful-tick hooks to
+// reconnect.go's setNeedsRejoin/checkAndPerformRejoin (task 4.3) -- closing
+// the idle-period reconnect gap (task 4.4, Req 2 AC4/AC5): XCECLI_FF_CALL
+// (backing xcecli_heartbeat) never invokes the client library's rejoin
+// callback, so nothing else would set the pending-rejoin flag if this
+// heartbeat prober is the only traffic during a controller outage.
 func StartHeartbeat(ctx context.Context) {
-	go runHeartbeat(ctx, func(secs int) {
-		C.xcecli_heartbeat(gCtx.ctx, C.int(secs))
-	})
+	go runHeartbeat(ctx, func(secs int) int {
+		return int(C.xcecli_heartbeat(gCtx.ctx, C.int(secs)))
+	}, setNeedsRejoin, checkAndPerformRejoin)
 }
 
 // go_push_handler is called by the C library on every incoming request.