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/builder.go

@@ -0,0 +1,85 @@
+package host
+
+import (
+	"bytes"
+	"fmt"
+	"os/exec"
+	"sync"
+)
+
+// Builder compiles Go source to WASM.
+type Builder struct {
+	sourceDir  string
+	outputPath string
+
+	mu       sync.Mutex
+	building bool
+}
+
+// BuildResult contains the result of a build operation.
+type BuildResult struct {
+	Success bool
+	Output  string
+	Error   error
+}
+
+// NewBuilder creates a new WASM builder.
+// sourceDir is the directory containing Go source files.
+// outputPath is the target WASM file path.
+func NewBuilder(sourceDir, outputPath string) *Builder {
+	return &Builder{
+		sourceDir:  sourceDir,
+		outputPath: outputPath,
+	}
+}
+
+// Build compiles the Go source to WASM.
+func (b *Builder) Build() BuildResult {
+	b.mu.Lock()
+	if b.building {
+		b.mu.Unlock()
+		return BuildResult{
+			Success: false,
+			Output:  "Build already in progress",
+			Error:   fmt.Errorf("build already in progress"),
+		}
+	}
+	b.building = true
+	b.mu.Unlock()
+
+	defer func() {
+		b.mu.Lock()
+		b.building = false
+		b.mu.Unlock()
+	}()
+
+	cmd := exec.Command("go", "build", "-o", b.outputPath, b.sourceDir)
+	cmd.Env = append(cmd.Environ(), "GOOS=js", "GOARCH=wasm")
+
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	err := cmd.Run()
+	output := stdout.String() + stderr.String()
+
+	if err != nil {
+		return BuildResult{
+			Success: false,
+			Output:  output,
+			Error:   err,
+		}
+	}
+
+	return BuildResult{
+		Success: true,
+		Output:  output,
+	}
+}
+
+// IsBuilding returns true if a build is currently in progress.
+func (b *Builder) IsBuilding() bool {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.building
+}