builder: write HTML size report

This is not a big change over the existing size report with -size=full,
but it is a bit more readable.

More information will be added in subsequent commits.

builder/size-report.go

@@ -0,0 +1,56 @@
+package builder
+
+import (
+	_ "embed"
+	"fmt"
+	"html/template"
+	"os"
+)
+
+//go:embed size-report.html
+var sizeReportBase string
+
+func writeSizeReport(sizes *programSize, filename, pkgName string) error {
+	tmpl, err := template.New("report").Parse(sizeReportBase)
+	if err != nil {
+		return err
+	}
+
+	f, err := os.Create(filename)
+	if err != nil {
+		return fmt.Errorf("could not open report file: %w", err)
+	}
+	defer f.Close()
+
+	// Prepare data for the report.
+	type sizeLine struct {
+		Name string
+		Size *packageSize
+	}
+	programData := []sizeLine{}
+	for _, name := range sizes.sortedPackageNames() {
+		pkgSize := sizes.Packages[name]
+		programData = append(programData, sizeLine{
+			Name: name,
+			Size: pkgSize,
+		})
+	}
+	sizeTotal := map[string]uint64{
+		"code":   sizes.Code,
+		"rodata": sizes.ROData,
+		"data":   sizes.Data,
+		"bss":    sizes.BSS,
+		"flash":  sizes.Flash(),
+	}
+
+	// Write the report.
+	err = tmpl.Execute(f, map[string]any{
+		"pkgName":   pkgName,
+		"sizes":     programData,
+		"sizeTotal": sizeTotal,
+	})
+	if err != nil {
+		return fmt.Errorf("could not create report file: %w", err)
+	}
+	return nil
+}