feat: implement isolated model worker with auto-restart and hot-reload

Introduce a supervised child process for capture, VAD, and recognition,
enabling automatic restart on crash/hang and hot-reload via SIGHUP.
Add `--model-auto-restart`, `--model-timeout`, and
`--model-shutdown-timeout` options. Refactor `Run()` with lifecycle
hooks for proper process group cleanup. Update README with new
behavior and CLI flags.

src/app/worker_protocol.go

@@ -0,0 +1,152 @@
+package app
+
+import (
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/signal"
+	"sync"
+	"syscall"
+	"time"
+
+	"tea.chunkbyte.com/kato/captioneer/src/captions"
+	"tea.chunkbyte.com/kato/captioneer/src/config"
+)
+
+const (
+	modelWorkerEnvironment   = "CAPTIONEER_INTERNAL_MODEL_WORKER"
+	modelSettingsEnvironment = "CAPTIONEER_INTERNAL_MODEL_SETTINGS"
+	modelProtocolFD          = 3
+)
+
+const (
+	messageReady     = "ready"
+	messageBusy      = "busy"
+	messageIdle      = "idle"
+	messageCaption   = "caption"
+	messageFailure   = "failure"
+	messageHeartbeat = "heartbeat"
+)
+
+type workerMessage struct {
+	Type  string          `json:"type"`
+	Event *captions.Event `json:"event,omitempty"`
+	Error string          `json:"error,omitempty"`
+}
+
+type workerReporter struct {
+	mu      sync.Mutex
+	encoder *json.Encoder
+}
+
+func (r *workerReporter) send(message workerMessage) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.encoder.Encode(message)
+}
+
+type protocolSink struct {
+	reporter *workerReporter
+}
+
+func (s protocolSink) Publish(event captions.Event) error {
+	return s.reporter.send(workerMessage{Type: messageCaption, Event: &event})
+}
+
+func (protocolSink) Close() error { return nil }
+
+// IsModelWorker reports whether this process was created as Captioneer's
+// private inference worker. The environment marker is intentionally internal.
+func IsModelWorker() bool {
+	return os.Getenv(modelWorkerEnvironment) == "1"
+}
+
+func encodeWorkerSettings(settings config.Settings) (string, error) {
+	data, err := json.Marshal(settings)
+	if err != nil {
+		return "", err
+	}
+	return base64.RawStdEncoding.EncodeToString(data), nil
+}
+
+func decodeWorkerSettings(value string) (config.Settings, error) {
+	data, err := base64.RawStdEncoding.DecodeString(value)
+	if err != nil {
+		return config.Settings{}, fmt.Errorf("decode model worker settings: %w", err)
+	}
+	var settings config.Settings
+	if err := json.Unmarshal(data, &settings); err != nil {
+		return config.Settings{}, fmt.Errorf("parse model worker settings: %w", err)
+	}
+	if err := settings.Validate(); err != nil {
+		return config.Settings{}, fmt.Errorf("validate model worker settings: %w", err)
+	}
+	return settings, nil
+}
+
+// RunModelWorker owns all Sherpa/VAD resources in the supervised child. It
+// returns a process exit code so command entrypoints can call os.Exit.
+func RunModelWorker(diagnostics io.Writer) int {
+	protocol := os.NewFile(uintptr(modelProtocolFD), "captioneer-model-protocol")
+	if protocol == nil {
+		fmt.Fprintln(diagnostics, "captioneer: model worker protocol is unavailable")
+		return 1
+	}
+	defer protocol.Close()
+	syscall.CloseOnExec(modelProtocolFD)
+
+	settings, err := decodeWorkerSettings(os.Getenv(modelSettingsEnvironment))
+	if err != nil {
+		fmt.Fprintf(diagnostics, "captioneer: %v\n", err)
+		return 1
+	}
+
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer stop()
+
+	reporter := &workerReporter{encoder: json.NewEncoder(protocol)}
+	hooks := directHooks{
+		ready: func() error { return reporter.send(workerMessage{Type: messageReady}) },
+		busy:  func() error { return reporter.send(workerMessage{Type: messageBusy}) },
+		idle:  func() error { return reporter.send(workerMessage{Type: messageIdle}) },
+	}
+
+	heartbeatInterval := time.Second
+	if settings.ModelTimeout > 0 && settings.ModelTimeout < 4*heartbeatInterval {
+		heartbeatInterval = settings.ModelTimeout / 4
+		if heartbeatInterval <= 0 {
+			heartbeatInterval = time.Millisecond
+		}
+	}
+	heartbeatContext, stopHeartbeat := context.WithCancel(ctx)
+	heartbeatDone := make(chan struct{})
+	go func() {
+		defer close(heartbeatDone)
+		ticker := time.NewTicker(heartbeatInterval)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-heartbeatContext.Done():
+				return
+			case <-ticker.C:
+				if reporter.send(workerMessage{Type: messageHeartbeat}) != nil {
+					return
+				}
+			}
+		}
+	}()
+
+	err = runDirect(ctx, settings, protocolSink{reporter: reporter}, diagnostics, hooks)
+	stopHeartbeat()
+	<-heartbeatDone
+	if err == nil || errors.Is(err, context.Canceled) {
+		return 0
+	}
+	_ = reporter.send(workerMessage{Type: messageFailure, Error: err.Error()})
+	fmt.Fprintf(diagnostics, "captioneer: model worker: %v\n", err)
+	return 1
+}