Add ACME (RFC 8555) server and Go client library

Implements full ACME protocol support in Metacrypt:

- internal/acme: core types, JWS verification (ES256/384/512 + RS256),
  nonce store, per-mount handler, all RFC 8555 protocol endpoints,
  HTTP-01 and DNS-01 challenge validation, EAB management
- internal/server/acme.go: management REST routes (EAB create, config,
  list accounts/orders) + ACME protocol route dispatch
- proto/metacrypt/v1/acme.proto: ACMEService (CreateEAB, SetConfig,
  ListAccounts, ListOrders) — protocol endpoints are HTTP-only per RFC
- clients/go: new Go module with MCIAS-auth bootstrap, ACME account
  registration, certificate issuance/renewal, HTTP-01 and DNS-01
  challenge providers
- .claude/launch.json: dev server configuration

EAB is required for all account creation; MCIAS-authenticated users
obtain a single-use KID + HMAC-SHA256 key via POST /v1/acme/{mount}/eab.

clients/go/challenge.go

@@ -0,0 +1,65 @@
+package metacryptclient
+
+import "time"
+
+// HTTP01Provider handles HTTP-01 ACME challenge fulfillment.
+// The caller is responsible for serving the challenge file over HTTP on port 80.
+type HTTP01Provider struct {
+	// WriteFile is called to create the challenge response file.
+	// path is the URL path, e.g. "/.well-known/acme-challenge/{token}".
+	// content is the key authorization string to serve as the response body.
+	WriteFile func(path, content string) error
+
+	// RemoveFile is called after the challenge is complete (success or failure).
+	// path is the same path passed to WriteFile.
+	RemoveFile func(path string) error
+}
+
+// DNS01Provider handles DNS-01 ACME challenge fulfillment.
+// The caller is responsible for managing DNS TXT records.
+type DNS01Provider struct {
+	// SetRecord is called to create the TXT record.
+	// name is the fully-qualified DNS name, e.g. "_acme-challenge.example.com.".
+	// value is the base64url-encoded SHA-256 key authorization to set as the TXT value.
+	SetRecord func(name, value string) error
+
+	// RemoveRecord is called after the challenge is complete (success or failure).
+	// name is the same name passed to SetRecord.
+	RemoveRecord func(name string) error
+
+	// PropagationWait is the duration to wait after SetRecord before notifying
+	// the ACME server to validate. Allows time for DNS propagation.
+	// A value of 0 means no wait.
+	PropagationWait time.Duration
+}
+
+// CertificateRequest specifies what certificate to obtain.
+type CertificateRequest struct {
+	// Domains is the list of DNS names to include as SANs.
+	// The first domain is used as the common name.
+	Domains []string
+
+	// KeyType controls the type and size of the certificate key.
+	// Valid values: "EC256" (default), "EC384", "RSA2048", "RSA4096".
+	KeyType string
+
+	// HTTP01 provides HTTP-01 challenge fulfillment. Either HTTP01 or DNS01
+	// (or both) must be set.
+	HTTP01 *HTTP01Provider
+
+	// DNS01 provides DNS-01 challenge fulfillment. Either HTTP01 or DNS01
+	// (or both) must be set.
+	DNS01 *DNS01Provider
+}
+
+// Certificate is the result of a successful ACME certificate issuance.
+type Certificate struct {
+	// CertPEM is the PEM-encoded certificate chain (leaf + intermediates).
+	CertPEM []byte
+
+	// KeyPEM is the PEM-encoded private key for the certificate.
+	KeyPEM []byte
+
+	// ExpiresAt is the expiry time of the leaf certificate.
+	ExpiresAt time.Time
+}