fix(worker): graceful job kill and catchable stop, as LSF (bug-1957)

A plain kill sent one SIGTERM and never escalated, and a plain stop sent
one uncatchable SIGSTOP. Following LSF's TERMINATE and SUSPEND actions, a
plain kill now sends SIGINT, then SIGTERM, then SIGKILL, and a plain stop
sends SIGTSTP (the job can catch it, prepare and stop itself), then SIGSTOP
if it has not stopped; each next signal follows after the grace period and
only while the job is alive (or not yet stopped). A resume cancels a
pending stop; the job's exit ends any sequence. An explicit signal is
delivered once; a force kill is SIGKILL at once.

- JobSignal gains grace (capnp @2, xce_signal_t.grace): > 0 asks for the
  sequence, in seconds between signals.
- apiserver: job.terminate_interval (default 10 s, LSF's
  JOB_TERMINATE_INTERVAL); jobCtrlParams sends kill = SIGKILL and stop =
  SIGSTOP with that grace when no signal is given; the handler no longer
  pre-fills a default signal.
- worker: grace travels through the job-control task, the kept-signal table
  (pending_signal_*_ex) and the IPC message; the job process runs the
  sequence (job_signal_seq_*, uv timer) and checks stop state in /proc.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/frontend/apiserver/utils/config.go

@@ -94,6 +94,10 @@ type FileUploadConfig struct {
 
 type JobConfig struct {
 	MaxArraySize int `yaml:"max_array_size"`
+	// TerminateInterval is the seconds between the signals of a plain kill
+	// (SIGINT, SIGTERM, SIGKILL) and a plain stop (SIGTSTP, SIGSTOP), as
+	// LSF's JOB_TERMINATE_INTERVAL (bug-1957)
+	TerminateInterval int `yaml:"terminate_interval"`
 }
 
 type CORSConfig struct {
@@ -239,8 +243,13 @@ func InitConf() error {
 			zap.Int("configured", configuredArraySize),
 			zap.Int("effective", maxArraySize))
 	}
+	terminateInterval := viper.GetInt("job.terminate_interval")
+	if terminateInterval < 0 {
+		return fmt.Errorf("job.terminate_interval must not be negative: %d", terminateInterval)
+	}
 	JobYamlConfig = &JobConfig{
-		MaxArraySize: maxArraySize,
+		MaxArraySize:      maxArraySize,
+		TerminateInterval: terminateInterval,
 	}
 
 	// parse cache config section