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>

proofstream.go

@@ -439,6 +439,49 @@ func sameString(a, b any) bool {
 	return a == nil && b == nil
 }
 
+func webhookHeader(headers map[string]string, name string) (string, bool) {
+	for key, value := range headers {
+		if strings.EqualFold(key, name) {
+			return value, true
+		}
+	}
+	return "", false
+}
+
+// VerifyWebhook verifies an inbound webhook delivered by Attesto. It reads the
+// X-Attesto-Timestamp / X-Attesto-Signature headers (case-insensitive), rejects
+// when the timestamp is outside maxSkewS of now, recomputes
+// hmac_sha256(secret, "<ts>." + body), and compares in constant time. Mirrors
+// the backend webhook signing scheme.
+func VerifyWebhook(body []byte, headers map[string]string, secret string, maxSkewS int) bool {
+	return verifyWebhookAt(body, headers, secret, maxSkewS, time.Now().Unix())
+}
+
+func verifyWebhookAt(body []byte, headers map[string]string, secret string, maxSkewS int, now int64) bool {
+	timestamp, okTS := webhookHeader(headers, "x-attesto-timestamp")
+	signature, okSig := webhookHeader(headers, "x-attesto-signature")
+	if !okTS || !okSig {
+		return false
+	}
+	ts, err := strconv.ParseInt(strings.TrimSpace(timestamp), 10, 64)
+	if err != nil {
+		return false
+	}
+	skew := now - ts
+	if skew < 0 {
+		skew = -skew
+	}
+	if skew > int64(maxSkewS) {
+		return false
+	}
+	mac := hmac.New(sha256.New, []byte(secret))
+	mac.Write([]byte(strconv.FormatInt(ts, 10)))
+	mac.Write([]byte("."))
+	mac.Write(body)
+	expected := hex.EncodeToString(mac.Sum(nil))
+	return hmac.Equal([]byte(expected), []byte(signature))
+}
+
 func SignConnectorWebhookPayload(secret string, body []byte, timestamp int64) (string, string) {
 	if timestamp == 0 {
 		timestamp = time.Now().Unix()