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.

internal/server/server.go

@@ -7,10 +7,12 @@ import (
 	"fmt"
 	"log/slog"
 	"net/http"
+	"sync"
 	"time"
 
 	"github.com/go-chi/chi/v5"
 
+	internacme "git.wntrmute.dev/kyle/metacrypt/internal/acme"
 	"git.wntrmute.dev/kyle/metacrypt/internal/auth"
 	"git.wntrmute.dev/kyle/metacrypt/internal/config"
 	"git.wntrmute.dev/kyle/metacrypt/internal/engine"
@@ -28,6 +30,9 @@ type Server struct {
 	httpSrv  *http.Server
 	logger   *slog.Logger
 	version  string
+
+	acmeMu       sync.Mutex
+	acmeHandlers map[string]*internacme.Handler
 }
 
 // New creates a new server.