Fix ECDH zeroization, add audit logging, and remediate high findings

- Fix #61: handleRotateKey and handleDeleteUser now zeroize stored
  privBytes instead of calling Bytes() (which returns a copy). New
  state populates privBytes; old references nil'd for GC.
- Add audit logging subsystem (internal/audit) with structured event
  recording for cryptographic operations.
- Add audit log engine spec (engines/auditlog.md).
- Add ValidateName checks across all engines for path traversal (#48).
- Update AUDIT.md: all High findings resolved (0 open).
- Add REMEDIATION.md with detailed remediation tracking.

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

cmd/metacrypt/init.go

@@ -51,7 +51,7 @@ func runInit(cmd *cobra.Command, args []string) error {
 
 	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
 	b := barrier.NewAESGCMBarrier(database)
-	sealMgr := seal.NewManager(database, b, logger)
+	sealMgr := seal.NewManager(database, b, nil, logger)
 	if err := sealMgr.CheckInitialized(); err != nil {
 		return err
 	}

cmd/metacrypt/server.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"fmt"
 	"log/slog"
 	"os"
 	"os/signal"
@@ -10,6 +11,7 @@ import (
 	mcias "git.wntrmute.dev/kyle/mcias/clients/go"
 	"github.com/spf13/cobra"
 
+	"git.wntrmute.dev/kyle/metacrypt/internal/audit"
 	"git.wntrmute.dev/kyle/metacrypt/internal/auth"
 	"git.wntrmute.dev/kyle/metacrypt/internal/barrier"
 	"git.wntrmute.dev/kyle/metacrypt/internal/config"
@@ -59,8 +61,14 @@ func runServer(cmd *cobra.Command, args []string) error {
 		return err
 	}
 
+	// Create audit logger.
+	auditLog, err := createAuditLogger(cfg)
+	if err != nil {
+		return err
+	}
+
 	b := barrier.NewAESGCMBarrier(database)
-	sealMgr := seal.NewManager(database, b, logger)
+	sealMgr := seal.NewManager(database, b, auditLog, logger)
 
 	if err := sealMgr.CheckInitialized(); err != nil {
 		return err
@@ -81,8 +89,8 @@ func runServer(cmd *cobra.Command, args []string) error {
 	engineRegistry.RegisterFactory(engine.EngineTypeTransit, transit.NewTransitEngine)
 	engineRegistry.RegisterFactory(engine.EngineTypeUser, user.NewUserEngine)
 
-	srv := server.New(cfg, sealMgr, authenticator, policyEngine, engineRegistry, logger, version)
-	grpcSrv := grpcserver.New(cfg, sealMgr, authenticator, policyEngine, engineRegistry, logger)
+	srv := server.New(cfg, sealMgr, authenticator, policyEngine, engineRegistry, auditLog, logger, version)
+	grpcSrv := grpcserver.New(cfg, sealMgr, authenticator, policyEngine, engineRegistry, auditLog, logger)
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
@@ -106,3 +114,26 @@ func runServer(cmd *cobra.Command, args []string) error {
 	grpcSrv.Shutdown()
 	return srv.Shutdown(context.Background())
 }
+
+func createAuditLogger(cfg *config.Config) (*audit.Logger, error) {
+	switch cfg.Audit.Mode {
+	case "":
+		return nil, nil // disabled
+	case "stdout":
+		h := slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
+			Level: audit.LevelAudit,
+		})
+		return audit.New(h), nil
+	case "file":
+		f, err := os.OpenFile(cfg.Audit.Path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0600)
+		if err != nil {
+			return nil, fmt.Errorf("audit: open log file %s: %w", cfg.Audit.Path, err)
+		}
+		h := slog.NewJSONHandler(f, &slog.HandlerOptions{
+			Level: audit.LevelAudit,
+		})
+		return audit.New(h), nil
+	default:
+		return nil, fmt.Errorf("audit: unknown mode %q", cfg.Audit.Mode)
+	}
+}