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>

internal/seal/seal.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 	"time"
 
+	"git.wntrmute.dev/kyle/metacrypt/internal/audit"
 	"git.wntrmute.dev/kyle/metacrypt/internal/barrier"
 	"git.wntrmute.dev/kyle/metacrypt/internal/crypto"
 )
@@ -54,6 +55,7 @@ type Manager struct {
 	lockoutUntil   time.Time
 	db             *sql.DB
 	barrier        *barrier.AESGCMBarrier
+	audit          *audit.Logger
 	logger         *slog.Logger
 	mek            []byte
 	state          ServiceState
@@ -62,10 +64,11 @@ type Manager struct {
 }
 
 // NewManager creates a new seal manager.
-func NewManager(db *sql.DB, b *barrier.AESGCMBarrier, logger *slog.Logger) *Manager {
+func NewManager(db *sql.DB, b *barrier.AESGCMBarrier, auditLog *audit.Logger, logger *slog.Logger) *Manager {
 	return &Manager{
 		db:      db,
 		barrier: b,
+		audit:   auditLog,
 		logger:  logger,
 		state:   StateUninitialized,
 	}
@@ -223,6 +226,10 @@ func (m *Manager) Unseal(password []byte) error {
 	mek, err := crypto.Decrypt(kwk, encryptedMEK, nil)
 	if err != nil {
 		m.logger.Debug("unseal failed: invalid password")
+		m.audit.Log(context.Background(), audit.Event{
+			Caller: "operator", Operation: "unseal", Outcome: "denied",
+			Error: "invalid password",
+		})
 		return ErrInvalidPassword
 	}
 
@@ -235,6 +242,9 @@ func (m *Manager) Unseal(password []byte) error {
 	m.mek = mek
 	m.state = StateUnsealed
 	m.unsealAttempts = 0
+	m.audit.Log(context.Background(), audit.Event{
+		Caller: "operator", Operation: "unseal", Outcome: "success",
+	})
 	m.logger.Debug("unseal succeeded, barrier unsealed")
 	return nil
 }
@@ -340,6 +350,9 @@ func (m *Manager) Seal() error {
 	}
 	_ = m.barrier.Seal()
 	m.state = StateSealed
+	m.audit.Log(context.Background(), audit.Event{
+		Caller: "operator", Operation: "seal", Outcome: "success",
+	})
 	m.logger.Debug("service sealed")
 	return nil
 }