sdk(P1.4): inbound webhook verification helper in all three SDKs

Customers receiving Attesto webhook deliveries can now verify them with one
call, mirroring backend/app/services/webhooks/signing.py exactly: read the
X-Attesto-Timestamp / X-Attesto-Signature headers (case-insensitive), reject
when abs(now - ts) > max_skew_s (300 s default, replay protection), recompute
hex(hmac_sha256(secret, f"{timestamp}.{body}")), constant-time compare
(hmac.compare_digest / charcode-XOR fold over equal-length hex / hmac.Equal).

- Python: attesto.verify_webhook(body=, headers=, secret=, max_skew_s=, now=)
- TypeScript: verifyWebhook({ body, headers, secret, maxSkewS, now }) via
  WebCrypto HMAC (edge-safe)
- Go: VerifyWebhook(body, headers, secret, maxSkewS)

New corpus golden-vectors/sdk-parity/webhook.json (valid, within-skew,
skewed-timestamp, bad-signature, tampered-body, wrong-secret,
non-numeric-timestamp) with backend-derived signatures; all three SDKs agree
on every case. READMEs gain a "Receiving Attesto webhooks" example.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

webhook_parity_test.go

@@ -0,0 +1,65 @@
+package attesto
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+type webhookParityVectors struct {
+	Cases []struct {
+		ID       string            `json:"id"`
+		Secret   string            `json:"secret"`
+		Body     string            `json:"body"`
+		Headers  map[string]string `json:"headers"`
+		Now      int64             `json:"now"`
+		MaxSkewS int               `json:"max_skew_s"`
+		ExpectOK bool              `json:"expect_ok"`
+	} `json:"cases"`
+}
+
+func TestParityWebhookVerification(t *testing.T) {
+	path := filepath.Join("..", "..", "golden-vectors", "sdk-parity", "webhook.json")
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read webhook vectors: %v", err)
+	}
+	var v webhookParityVectors
+	if err := json.Unmarshal(raw, &v); err != nil {
+		t.Fatalf("decode webhook vectors: %v", err)
+	}
+	for _, c := range v.Cases {
+		ok := verifyWebhookAt([]byte(c.Body), c.Headers, c.Secret, c.MaxSkewS, c.Now)
+		if ok != c.ExpectOK {
+			t.Errorf("webhook %s: ok=%v want %v", c.ID, ok, c.ExpectOK)
+		}
+	}
+}
+
+func TestWebhookHeaderLookupIsCaseInsensitive(t *testing.T) {
+	path := filepath.Join("..", "..", "golden-vectors", "sdk-parity", "webhook.json")
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read webhook vectors: %v", err)
+	}
+	var v webhookParityVectors
+	if err := json.Unmarshal(raw, &v); err != nil {
+		t.Fatalf("decode webhook vectors: %v", err)
+	}
+	for _, c := range v.Cases {
+		if c.ID != "valid" {
+			continue
+		}
+		lowered := map[string]string{}
+		for key, value := range c.Headers {
+			lowered[key] = value
+		}
+		// Re-key with different casing.
+		lowered["X-ATTESTO-TIMESTAMP"] = lowered["X-Attesto-Timestamp"]
+		delete(lowered, "X-Attesto-Timestamp")
+		if !verifyWebhookAt([]byte(c.Body), lowered, c.Secret, c.MaxSkewS, c.Now) {
+			t.Error("case-insensitive header lookup failed")
+		}
+	}
+}