feat(bundler): esbuild wrapper for spec compilation

Bundles a TypeScript entry into an IIFE ES2020 blob with optional
inline sourcemaps. process.env defines are JSON-quoted before being
fed to esbuild so values with quotes/backslashes survive correctly.
Returns the bundle's SHA-256 for trace meta.json.

internal/bundler/bundler.go

@@ -0,0 +1,76 @@
+package bundler
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+
+	esbuild "github.com/evanw/esbuild/pkg/api"
+)
+
+type Options struct {
+	EntryFile string
+	Defines   map[string]string
+	Sourcemap bool
+}
+
+type Result struct {
+	JavaScript []byte
+	SHA256     string
+}
+
+// Bundle compiles the entry TypeScript file into a single IIFE JavaScript
+// blob targeting ES2020. Defines are injected as literal string values for
+// process.env.<NAME> lookups in the spec source.
+func Bundle(options Options) (Result, error) {
+	if options.EntryFile == "" {
+		return Result{}, errors.New("EntryFile is required")
+	}
+
+	defines := map[string]string{}
+	for key, value := range options.Defines {
+		quoted, err := json.Marshal(value)
+		if err != nil {
+			return Result{}, fmt.Errorf("define %q: %w", key, err)
+		}
+		defines["process.env."+key] = string(quoted)
+	}
+
+	sourcemap := esbuild.SourceMapNone
+	if options.Sourcemap {
+		sourcemap = esbuild.SourceMapInline
+	}
+
+	output := esbuild.Build(esbuild.BuildOptions{
+		EntryPoints: []string{options.EntryFile},
+		Bundle:      true,
+		Format:      esbuild.FormatIIFE,
+		Target:      esbuild.ES2020,
+		Platform:    esbuild.PlatformNeutral,
+		Define:      defines,
+		Sourcemap:   sourcemap,
+		Write:       false,
+		LogLevel:    esbuild.LogLevelSilent,
+	})
+
+	if len(output.Errors) > 0 {
+		var messages []string
+		for _, err := range output.Errors {
+			messages = append(messages, err.Text)
+		}
+		return Result{}, fmt.Errorf("bundle failed: %s", strings.Join(messages, "; "))
+	}
+	if len(output.OutputFiles) == 0 {
+		return Result{}, errors.New("bundle produced no output files")
+	}
+
+	javascript := output.OutputFiles[0].Contents
+	sum := sha256.Sum256(javascript)
+	return Result{
+		JavaScript: javascript,
+		SHA256:     hex.EncodeToString(sum[:]),
+	}, nil
+}