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/server/server_test.go

@@ -36,7 +36,7 @@ func setupTestServer(t *testing.T) (*Server, *seal.Manager, chi.Router) {
 	_ = db.Migrate(database)
 
 	b := barrier.NewAESGCMBarrier(database)
-	sealMgr := seal.NewManager(database, b, slog.Default())
+	sealMgr := seal.NewManager(database, b, nil, slog.Default())
 	_ = sealMgr.CheckInitialized()
 
 	// Auth requires MCIAS client which we can't create in tests easily,
@@ -61,7 +61,7 @@ func setupTestServer(t *testing.T) (*Server, *seal.Manager, chi.Router) {
 	}
 
 	logger := slog.Default()
-	srv := New(cfg, sealMgr, authenticator, policyEngine, engineRegistry, logger, "test")
+	srv := New(cfg, sealMgr, authenticator, policyEngine, engineRegistry, nil, logger, "test")
 
 	r := chi.NewRouter()
 	srv.registerRoutes(r)