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/engine/user/user.go

@@ -212,6 +212,9 @@ func (e *UserEngine) handleRegister(ctx context.Context, req *engine.Request) (*
 	}
 
 	username := req.CallerInfo.Username
+	if err := engine.ValidateName(username); err != nil {
+		return nil, fmt.Errorf("user: invalid username: %w", err)
+	}
 	e.mu.RLock()
 	if u, ok := e.users[username]; ok {
 		pubB64 := base64.StdEncoding.EncodeToString(u.pubKey.Bytes())
@@ -302,6 +305,9 @@ func (e *UserEngine) handleGetPublicKey(_ context.Context, req *engine.Request)
 	if username == "" {
 		return nil, fmt.Errorf("user: username is required")
 	}
+	if err := engine.ValidateName(username); err != nil {
+		return nil, err
+	}
 
 	e.mu.RLock()
 	defer e.mu.RUnlock()
@@ -657,14 +663,16 @@ func (e *UserEngine) handleRotateKey(ctx context.Context, req *engine.Request) (
 		return nil, fmt.Errorf("user: rotate key: %w", err)
 	}
 
-	// Zeroize old key.
-	oldRaw := oldState.privKey.Bytes()
-	crypto.Zeroize(oldRaw)
+	// Zeroize old key material and drop reference for GC.
+	crypto.Zeroize(oldState.privBytes)
+	oldState.privKey = nil
+	oldState.privBytes = nil
 
 	// Update in-memory state.
 	e.users[caller] = &userState{
-		privKey: priv,
-		pubKey:  priv.PublicKey(),
+		privKey:   priv,
+		privBytes: priv.Bytes(),
+		pubKey:    priv.PublicKey(),
 		config: &UserKeyConfig{
 			Algorithm:       e.config.KeyAlgorithm,
 			CreatedAt:       time.Now().UTC(),
@@ -692,6 +700,9 @@ func (e *UserEngine) handleDeleteUser(ctx context.Context, req *engine.Request)
 	if username == "" {
 		return nil, fmt.Errorf("user: username is required")
 	}
+	if err := engine.ValidateName(username); err != nil {
+		return nil, err
+	}
 
 	e.mu.Lock()
 	defer e.mu.Unlock()
@@ -701,9 +712,10 @@ func (e *UserEngine) handleDeleteUser(ctx context.Context, req *engine.Request)
 		return nil, ErrUserNotFound
 	}
 
-	// Zeroize private key.
-	oldRaw := oldState.privKey.Bytes()
-	crypto.Zeroize(oldRaw)
+	// Zeroize private key material and drop reference for GC.
+	crypto.Zeroize(oldState.privBytes)
+	oldState.privKey = nil
+	oldState.privBytes = nil
 
 	// Delete from barrier.
 	prefix := e.mountPath + "users/" + username + "/"