Implement CA/PKI engine with two-tier X.509 certificate issuance

Add the first concrete engine implementation: a CA (PKI) engine that generates
a self-signed root CA at mount time, issues scoped intermediate CAs ("issuers"),
and signs leaf certificates using configurable profiles (server, client, peer).

Engine framework updates:
- Add CallerInfo struct for auth context in engine requests
- Add config parameter to Engine.Initialize for mount-time configuration
- Export Mount.Engine field; add GetEngine/GetMount on Registry

CA engine (internal/engine/ca/):
- Two-tier PKI: root CA → issuers → leaf certificates
- 10 operations: get-root, get-chain, get-issuer, create/delete/list issuers,
  issue, get-cert, list-certs, renew
- Certificate profiles with user-overridable TTL, key usages, and key algorithm
- Private keys never stored in barrier; zeroized from memory on seal
- Supports ECDSA, RSA, and Ed25519 key types via goutils/certlib/certgen

Server routes:
- Wire up engine mount/request handlers (replace Phase 1 stubs)
- Add public PKI routes (/v1/pki/{mount}/ca, /ca/chain, /issuer/{name})
  for unauthenticated TLS trust bootstrapping

Also includes: ARCHITECTURE.md, deploy config updates, operational tooling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/docker/docker-compose.yml

@@ -8,13 +8,13 @@ services:
     ports:
       - "8443:8443"
     volumes:
-      - metacrypt-data:/data
-    # To populate /data before first run, use an init container or
+      - metacrypt-data:/srv/metacrypt
+    # To populate /srv/metacrypt before first run, use an init container or
     # bind-mount a host directory instead of a named volume:
     #   volumes:
-    #     - ./data:/data
+    #     - ./data:/srv/metacrypt
     healthcheck:
-      test: ["CMD", "metacrypt", "status", "--addr", "https://localhost:8443", "--ca-cert", "/data/certs/ca.crt"]
+      test: ["CMD", "metacrypt", "status", "--addr", "https://localhost:8443", "--ca-cert", "/srv/metacrypt/certs/ca.crt"]
       interval: 30s
       timeout: 5s
       retries: 3