feat(wasm): add registerComponents export for WC codegen

Exposes gohtml.registerComponents(slotsJSON) to browser context.
Pure-Go buildComponentJS is testable without WASM; WASM glue uses
Function constructor for browser-side execution.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/wasm/main.go

@@ -50,9 +50,28 @@ func renderToString(_ js.Value, args []js.Value) any {
 	return layout.Render(ctx)
 }
 
+// registerComponentsJS wraps buildComponentJS for the WASM→JS bridge.
+// Takes a JSON string of slot assignments, generates the WC bundle,
+// and executes it in the browser via the Function constructor.
+func registerComponentsJS(_ js.Value, args []js.Value) any {
+	if len(args) < 1 {
+		return js.ValueOf("error: slotsJSON argument required")
+	}
+	jsCode, err := buildComponentJS(args[0].String())
+	if err != nil {
+		return js.ValueOf("error: " + err.Error())
+	}
+	// Execute the generated WC definitions in the browser context.
+	// Uses the standard Function constructor — the normal Go WASM→JS pattern.
+	fn := js.Global().Call("Function", jsCode)
+	fn.Invoke()
+	return js.ValueOf(jsCode)
+}
+
 func main() {
 	js.Global().Set("gohtml", js.ValueOf(map[string]any{
-		"renderToString": js.FuncOf(renderToString),
+		"renderToString":     js.FuncOf(renderToString),
+		"registerComponents": js.FuncOf(registerComponentsJS),
 	}))
 
 	select {}

cmd/wasm/register.go

@@ -0,0 +1,18 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"forge.lthn.ai/core/go-html/codegen"
+)
+
+// buildComponentJS takes a JSON slot map and returns the WC bundle JS string.
+// This is the pure-Go part testable without WASM.
+func buildComponentJS(slotsJSON string) (string, error) {
+	var slots map[string]string
+	if err := json.Unmarshal([]byte(slotsJSON), &slots); err != nil {
+		return "", fmt.Errorf("registerComponents: %w", err)
+	}
+	return codegen.GenerateBundle(slots)
+}

cmd/wasm/register_test.go

@@ -0,0 +1,24 @@
+//go:build !js
+
+package main
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBuildComponentJS_Good(t *testing.T) {
+	slotsJSON := `{"H":"nav-bar","C":"main-content"}`
+	js, err := buildComponentJS(slotsJSON)
+	require.NoError(t, err)
+	assert.Contains(t, js, "NavBar")
+	assert.Contains(t, js, "MainContent")
+	assert.Contains(t, js, "customElements.define")
+}
+
+func TestBuildComponentJS_Bad_InvalidJSON(t *testing.T) {
+	_, err := buildComponentJS("not json")
+	assert.Error(t, err)
+}