mavgpud: a yield stops writing a core and reads as a yield (V-491)

llama-server aborts inside its own static teardown on SIGTERM — the
handler calls exit(), stream_session_manager's destructor throws, and the
process dies "signal: aborted (core dumped)". mavgpud sends that signal on
every eviction, so a routine yield wrote a multi-gigabyte core into
systemd-coredump and logged the same line a real crash would.

LimitCORE=0 in the unit stops the disk cost. A yielding flag, set by stop
and cleared by start, makes the log distinguish the two: only an exit we
did not ask for is still reported as an exit.

Not filed upstream. Searched ggml-org/llama.cpp for
"ggml_uncaught_exception" with SIGTERM and for stream_session_manager and
found nothing matching, so the issue still wants writing — by someone with
an account on that tracker, which is why it is not in this commit.

cmd/mavgpud/runner.go

@@ -24,7 +24,12 @@ type runner struct {
 	mu    sync.Mutex
 	cmd   *exec.Cmd
 	ready bool
-	http  *http.Client
+	// yielding — stop() has sent the signal and the exit that follows is ours.
+	// llama-server aborts on SIGTERM (its static teardown throws, upstream
+	// ggml-org/llama.cpp), so a routine yield and a real crash produce the same
+	// "signal: aborted" and used to log identically (Vikunja #491).
+	yielding bool
+	http     *http.Client
 }
 
 func newRunner(bin string, args []string, readyURL string) *runner {
@@ -70,13 +75,18 @@ func (r *runner) start() error {
 	if err := cmd.Start(); err != nil {
 		return err
 	}
-	r.cmd, r.ready = cmd, false
+	r.cmd, r.ready, r.yielding = cmd, false, false
 	log.Printf("mavgpud: started llama-server pid=%d", cmd.Process.Pid)
 	go func() {
 		err := cmd.Wait()
 		r.mu.Lock()
-		r.cmd, r.ready = nil, false
+		yielded := r.yielding
+		r.cmd, r.ready, r.yielding = nil, false, false
 		r.mu.Unlock()
+		if yielded {
+			log.Printf("mavgpud: llama-server stopped, card yielded (%v)", err)
+			return
+		}
 		log.Printf("mavgpud: llama-server exited: %v", err)
 	}()
 	return nil
@@ -90,6 +100,10 @@ func (r *runner) stop(grace time.Duration) {
 	r.mu.Lock()
 	cmd := r.cmd
 	r.ready = false
+	if cmd != nil && cmd.Process != nil {
+		// The exit that follows is ours, not a crash.
+		r.yielding = true
+	}
 	r.mu.Unlock()
 	if cmd == nil || cmd.Process == nil {
 		return