Add hot reload for development

Implement automatic rebuild and browser reload during development:

- File watcher monitors .go files for changes with configurable extensions
- Builder compiles Go source to WASM on file changes
- LiveReload WebSocket server notifies connected browsers to reload
- DevServer combines all components for easy development setup
- HTML injection adds reload script automatically

Usage:
  dev := host.NewDevServer("public", "index.html", ".", "public/app.wasm")
  dev.ListenAndServe(":8080")

Closes #9

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

host/watcher_test.go

@@ -0,0 +1,216 @@
+package host
+
+import (
+	"os"
+	"path/filepath"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestWatcher_DetectsNewFiles(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create initial file
+	initialFile := filepath.Join(dir, "initial.go")
+	if err := os.WriteFile(initialFile, []byte("package main"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	var changeCount int32
+	w := NewWatcher(dir, func() {
+		atomic.AddInt32(&changeCount, 1)
+	}, WithInterval(50*time.Millisecond))
+
+	if err := w.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer w.Stop()
+
+	// Create new file
+	newFile := filepath.Join(dir, "new.go")
+	if err := os.WriteFile(newFile, []byte("package main"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Wait for detection
+	time.Sleep(200 * time.Millisecond)
+
+	if count := atomic.LoadInt32(&changeCount); count == 0 {
+		t.Error("Expected change to be detected for new file")
+	}
+}
+
+func TestWatcher_DetectsModifiedFiles(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create initial file
+	file := filepath.Join(dir, "test.go")
+	if err := os.WriteFile(file, []byte("package main"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	var changeCount int32
+	w := NewWatcher(dir, func() {
+		atomic.AddInt32(&changeCount, 1)
+	}, WithInterval(50*time.Millisecond))
+
+	if err := w.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer w.Stop()
+
+	// Ensure mod time will be different
+	time.Sleep(100 * time.Millisecond)
+
+	// Modify file
+	if err := os.WriteFile(file, []byte("package main\n// updated"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Wait for detection
+	time.Sleep(200 * time.Millisecond)
+
+	if count := atomic.LoadInt32(&changeCount); count == 0 {
+		t.Error("Expected change to be detected for modified file")
+	}
+}
+
+func TestWatcher_DetectsDeletedFiles(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create initial file
+	file := filepath.Join(dir, "test.go")
+	if err := os.WriteFile(file, []byte("package main"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	var changeCount int32
+	w := NewWatcher(dir, func() {
+		atomic.AddInt32(&changeCount, 1)
+	}, WithInterval(50*time.Millisecond))
+
+	if err := w.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer w.Stop()
+
+	// Delete file
+	if err := os.Remove(file); err != nil {
+		t.Fatal(err)
+	}
+
+	// Wait for detection
+	time.Sleep(200 * time.Millisecond)
+
+	if count := atomic.LoadInt32(&changeCount); count == 0 {
+		t.Error("Expected change to be detected for deleted file")
+	}
+}
+
+func TestWatcher_IgnoresNonMatchingExtensions(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create initial go file
+	goFile := filepath.Join(dir, "main.go")
+	if err := os.WriteFile(goFile, []byte("package main"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	var changeCount int32
+	w := NewWatcher(dir, func() {
+		atomic.AddInt32(&changeCount, 1)
+	}, WithInterval(50*time.Millisecond), WithExtensions(".go"))
+
+	if err := w.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer w.Stop()
+
+	// Create non-matching file
+	txtFile := filepath.Join(dir, "readme.txt")
+	if err := os.WriteFile(txtFile, []byte("readme"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Wait
+	time.Sleep(200 * time.Millisecond)
+
+	if count := atomic.LoadInt32(&changeCount); count != 0 {
+		t.Error("Expected no change detected for non-matching extension")
+	}
+}
+
+func TestWatcher_CustomExtensions(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create initial file
+	file := filepath.Join(dir, "style.css")
+	if err := os.WriteFile(file, []byte("body {}"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	var changeCount int32
+	w := NewWatcher(dir, func() {
+		atomic.AddInt32(&changeCount, 1)
+	}, WithInterval(50*time.Millisecond), WithExtensions(".css", ".html"))
+
+	if err := w.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer w.Stop()
+
+	// Ensure mod time will be different
+	time.Sleep(100 * time.Millisecond)
+
+	// Modify css file
+	if err := os.WriteFile(file, []byte("body { color: red; }"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Wait for detection
+	time.Sleep(200 * time.Millisecond)
+
+	if count := atomic.LoadInt32(&changeCount); count == 0 {
+		t.Error("Expected change to be detected for custom extension")
+	}
+}
+
+func TestWatcher_SkipsHiddenDirs(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create hidden directory with go file
+	hiddenDir := filepath.Join(dir, ".hidden")
+	if err := os.MkdirAll(hiddenDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create a visible go file first
+	visibleFile := filepath.Join(dir, "visible.go")
+	if err := os.WriteFile(visibleFile, []byte("package main"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	var changeCount int32
+	w := NewWatcher(dir, func() {
+		atomic.AddInt32(&changeCount, 1)
+	}, WithInterval(50*time.Millisecond))
+
+	if err := w.Start(); err != nil {
+		t.Fatal(err)
+	}
+	defer w.Stop()
+
+	// Create file in hidden directory
+	hiddenFile := filepath.Join(hiddenDir, "hidden.go")
+	if err := os.WriteFile(hiddenFile, []byte("package main"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Wait
+	time.Sleep(200 * time.Millisecond)
+
+	if count := atomic.LoadInt32(&changeCount); count != 0 {
+		t.Error("Expected no change detected for hidden directory")
+	}
+}