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/inject.go

@@ -0,0 +1,80 @@
+package host
+
+import (
+	"bytes"
+	"strings"
+)
+
+// ReloadScript is the JavaScript code injected into HTML pages for hot reload.
+const ReloadScript = `<script>
+(function() {
+    var wsUrl = 'ws://' + window.location.host + '/__livereload';
+    var ws;
+    var reconnectAttempts = 0;
+    var maxReconnectDelay = 5000;
+
+    function connect() {
+        ws = new WebSocket(wsUrl);
+        
+        ws.onopen = function() {
+            console.log('[iris] Hot reload connected');
+            reconnectAttempts = 0;
+        };
+        
+        ws.onmessage = function(event) {
+            if (event.data === 'reload') {
+                console.log('[iris] Reloading...');
+                window.location.reload();
+            }
+        };
+        
+        ws.onclose = function() {
+            var delay = Math.min(1000 * Math.pow(2, reconnectAttempts), maxReconnectDelay);
+            reconnectAttempts++;
+            console.log('[iris] Connection lost. Reconnecting in ' + delay + 'ms...');
+            setTimeout(connect, delay);
+        };
+        
+        ws.onerror = function() {
+            ws.close();
+        };
+    }
+
+    connect();
+})();
+</script>`
+
+// InjectReloadScript injects the live reload script into HTML content.
+// The script is inserted just before the closing </body> tag.
+func InjectReloadScript(html []byte) []byte {
+	// Try to inject before </body>
+	bodyEnd := bytes.LastIndex(html, []byte("</body>"))
+	if bodyEnd != -1 {
+		result := make([]byte, 0, len(html)+len(ReloadScript))
+		result = append(result, html[:bodyEnd]...)
+		result = append(result, []byte(ReloadScript)...)
+		result = append(result, html[bodyEnd:]...)
+		return result
+	}
+
+	// Try to inject before </html>
+	htmlEnd := bytes.LastIndex(html, []byte("</html>"))
+	if htmlEnd != -1 {
+		result := make([]byte, 0, len(html)+len(ReloadScript))
+		result = append(result, html[:htmlEnd]...)
+		result = append(result, []byte(ReloadScript)...)
+		result = append(result, html[htmlEnd:]...)
+		return result
+	}
+
+	// Append at the end as fallback
+	result := make([]byte, 0, len(html)+len(ReloadScript))
+	result = append(result, html...)
+	result = append(result, []byte(ReloadScript)...)
+	return result
+}
+
+// IsHTMLContent checks if the content type indicates HTML.
+func IsHTMLContent(contentType string) bool {
+	return strings.Contains(contentType, "text/html")
+}