feat(macos): PDF Services stamp helper with dual-channel Verae auth

Implements macOS v0: verae-print stamp + install-macos PDF menu items.
Saves {name}.pdf, {name}-verae-timestamp-{RECEIPTID}.pdf, and .verae.json.
Channels: verae (api.veraetime.net JWT) and zapier (zap.verae.ai xVerae- + attachPdf).

internal/pdfcert/pdfcert.go

@@ -0,0 +1,103 @@
+package pdfcert
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// Input for a simple one-page Verae registration certificate PDF.
+type Input struct {
+	DocumentName string
+	SHA256       string
+	ReceiptID    string
+	CreatedAt    string
+	Merkle       string
+	VerifyURL    string
+	Channel      string
+}
+
+// Build returns a minimal PDF (no external deps) describing the stamp.
+func Build(in Input) ([]byte, error) {
+	created := in.CreatedAt
+	if created == "" {
+		created = time.Now().UTC().Format(time.RFC3339)
+	}
+	local := ""
+	if t, err := time.Parse(time.RFC3339, created); err == nil {
+		local = t.Local().Format(time.RFC1123)
+	}
+	verify := in.VerifyURL
+	if verify == "" && in.SHA256 != "" {
+		verify = fmt.Sprintf("https://mcp.verae.ai/register.html?sha256=%s&receipt=%s", in.SHA256, in.ReceiptID)
+	}
+
+	lines := []string{
+		"Verae Timestamp Registration",
+		"",
+		"Document: " + sanitize(in.DocumentName),
+		"SHA-256: " + in.SHA256,
+		"Receipt ID: " + in.ReceiptID,
+		"UTC: " + created,
+	}
+	if local != "" {
+		lines = append(lines, "Local: "+local)
+	}
+	if in.Merkle != "" {
+		lines = append(lines, "Merkle/block: "+in.Merkle)
+	}
+	if in.Channel != "" {
+		lines = append(lines, "Channel: "+in.Channel)
+	}
+	lines = append(lines, "Verify: "+verify)
+	lines = append(lines, "", "Digests only — document bytes were not uploaded.")
+
+	return buildTextPDF(lines)
+}
+
+func sanitize(s string) string {
+	s = strings.ReplaceAll(s, "(", "\\(")
+	s = strings.ReplaceAll(s, ")", "\\)")
+	s = strings.ReplaceAll(s, "\\", "\\\\")
+	return s
+}
+
+// Minimal PDF 1.4 with Helvetica text lines.
+func buildTextPDF(lines []string) ([]byte, error) {
+	var content strings.Builder
+	content.WriteString("BT /F1 12 Tf 50 750 Td 14 TL\n")
+	for i, line := range lines {
+		if i > 0 {
+			content.WriteString("T*\n")
+		}
+		content.WriteString("(")
+		content.WriteString(sanitize(line))
+		content.WriteString(") Tj\n")
+	}
+	content.WriteString("ET\n")
+	stream := content.String()
+
+	objects := []string{
+		"<< /Type /Catalog /Pages 2 0 R >>",
+		"<< /Type /Pages /Kids [3 0 R] /Count 1 >>",
+		"<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Contents 4 0 R /Resources << /Font << /F1 5 0 R >> >> >>",
+		fmt.Sprintf("<< /Length %d >>\nstream\n%s\nendstream", len(stream), stream),
+		"<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica >>",
+	}
+
+	var body strings.Builder
+	body.WriteString("%PDF-1.4\n")
+	offsets := make([]int, len(objects)+1)
+	for i, obj := range objects {
+		offsets[i+1] = body.Len()
+		fmt.Fprintf(&body, "%d 0 obj\n%s\nendobj\n", i+1, obj)
+	}
+	xrefStart := body.Len()
+	fmt.Fprintf(&body, "xref\n0 %d\n", len(objects)+1)
+	body.WriteString("0000000000 65535 f \n")
+	for i := 1; i <= len(objects); i++ {
+		fmt.Fprintf(&body, "%010d 00000 n \n", offsets[i])
+	}
+	fmt.Fprintf(&body, "trailer << /Size %d /Root 1 0 R >>\nstartxref\n%d\n%%%%EOF\n", len(objects)+1, xrefStart)
+	return []byte(body.String()), nil
+}