test(wasm): add binary size gate test

TestWASMBinarySize builds WASM with -ldflags="-s -w", gzips the
binary, and asserts < 1MB gzip (842KB actual) and < 3MB raw (2.90MB
actual). Skipped in -short mode. Matches Makefile WASM_GZ_LIMIT.

Co-Authored-By: Virgil <virgil@lethean.io>

TODO.md

@@ -28,7 +28,7 @@ Root cause: `registerComponents()` pulled in `encoding/json` (~200KB gz), `text/
 
 ### Step 5: Tests
 
-- [ ] **WASM build gate test** — `TestWASMBinarySize` in `cmd/wasm/main_test.go`: build WASM, gzip, assert < 1MB
+- [x] **WASM build gate test** — `TestWASMBinarySize` in `cmd/wasm/size_test.go`: builds WASM, gzips, asserts < 1MB gzip and < 3MB raw. Result: 2.90MB raw, 842KB gzip. `//go:build !js` guarded.
 - [x] **Codegen CLI test** — `cmd/codegen/main_test.go`: pipe JSON stdin, verify JS output matches `GenerateBundle()`
 - [x] **renderToString still works** — Existing WASM tests for `renderToString` pass (build-tag guarded)
 - [x] **Existing tests still pass** — `go test ./...` (non-WASM) all 70+ tests pass, pipeline/codegen tests unaffected

cmd/wasm/size_test.go

@@ -0,0 +1,52 @@
+// SPDX-Licence-Identifier: EUPL-1.2
+//go:build !js
+
+package main
+
+import (
+	"bytes"
+	"compress/gzip"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	wasmGzLimit  = 1_048_576 // 1 MB gzip transfer size limit
+	wasmRawLimit = 3_145_728 // 3 MB raw size limit
+)
+
+func TestWASMBinarySize_Good(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping WASM build test in short mode")
+	}
+
+	dir := t.TempDir()
+	out := filepath.Join(dir, "gohtml.wasm")
+
+	cmd := exec.Command("go", "build", "-ldflags=-s -w", "-o", out, ".")
+	cmd.Env = append(os.Environ(), "GOOS=js", "GOARCH=wasm")
+	output, err := cmd.CombinedOutput()
+	require.NoError(t, err, "WASM build failed: %s", output)
+
+	raw, err := os.ReadFile(out)
+	require.NoError(t, err)
+
+	var buf bytes.Buffer
+	gz, err := gzip.NewWriterLevel(&buf, gzip.BestCompression)
+	require.NoError(t, err)
+	_, err = gz.Write(raw)
+	require.NoError(t, err)
+	require.NoError(t, gz.Close())
+
+	t.Logf("WASM size: %d bytes raw, %d bytes gzip", len(raw), buf.Len())
+
+	assert.Less(t, buf.Len(), wasmGzLimit,
+		"WASM gzip size %d exceeds 1MB limit", buf.Len())
+	assert.Less(t, len(raw), wasmRawLimit,
+		"WASM raw size %d exceeds 3MB limit", len(raw))
+}