feat(M43): Sectigo SCM issuer connector

Implement Sectigo Certificate Manager REST API connector with async
order model (enroll → poll → collect PEM), 3-header auth, DV/OV/EV
support, collect-not-ready (400/-183) graceful handling, and RFC 5280
revocation reason mapping. 20 tests with httptest mock API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/architecture.md

@@ -513,6 +513,7 @@ flowchart TB
         II --> OC["OpenSSL / Custom CA"]
         II --> VP["Vault PKI"]
         II --> DC["DigiCert CertCentral"]
+        II --> SG["Sectigo SCM"]
     end
 
     subgraph "Target Connectors"