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/supervisor_test.go

@@ -0,0 +1,66 @@
+package app
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"tea.chunkbyte.com/kato/captioneer/src/config"
+)
+
+func TestWorkerHealthDetectsStartupAndOperationTimeouts(t *testing.T) {
+	started := time.Unix(100, 0)
+	health := workerHealth{startedAt: started, lastSeen: started}
+
+	if err := health.timeoutError(started.Add(time.Second), 2*time.Second); err != nil {
+		t.Fatalf("healthy startup reported a timeout: %v", err)
+	}
+	if err := health.timeoutError(started.Add(2*time.Second), 2*time.Second); err == nil ||
+		!strings.Contains(err.Error(), "did not become ready") {
+		t.Fatalf("startup timeout = %v", err)
+	}
+
+	health.observe(messageReady, started.Add(2*time.Second))
+	health.observe(messageBusy, started.Add(3*time.Second))
+	if err := health.timeoutError(started.Add(4*time.Second), 2*time.Second); err != nil {
+		t.Fatalf("healthy operation reported a timeout: %v", err)
+	}
+	if err := health.timeoutError(started.Add(5*time.Second), 2*time.Second); err == nil ||
+		!strings.Contains(err.Error(), "operation exceeded") {
+		t.Fatalf("operation timeout = %v", err)
+	}
+
+	health.observe(messageIdle, started.Add(5*time.Second))
+	health.observe(messageHeartbeat, started.Add(6*time.Second))
+	if err := health.timeoutError(started.Add(7*time.Second), 2*time.Second); err != nil {
+		t.Fatalf("idle worker reported a timeout: %v", err)
+	}
+	if err := health.timeoutError(started.Add(8*time.Second), 2*time.Second); err == nil ||
+		!strings.Contains(err.Error(), "heartbeat was silent") {
+		t.Fatalf("heartbeat timeout = %v", err)
+	}
+}
+
+func TestWorkerSettingsRoundTrip(t *testing.T) {
+	want := config.Settings{
+		Mode:                 config.ModeVAD,
+		ChunkDuration:        time.Second,
+		ModelsDir:            "models with spaces",
+		Threads:              3,
+		PreviewThresholdDB:   -42,
+		ModelAutoRestart:     true,
+		ModelTimeout:         12 * time.Second,
+		ModelShutdownTimeout: 2 * time.Second,
+	}
+	encoded, err := encodeWorkerSettings(want)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got, err := decodeWorkerSettings(encoded)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != want {
+		t.Fatalf("settings round trip = %#v, want %#v", got, want)
+	}
+}