test(e2e): add playwright e2e harness

Process and API-only Playwright harness: binary and image setup, server/CLI runners, fake Docker Engine socket, read-only database inspection, secret redaction in reports, seeded fuzzing, leak detection and harness self-checks.

e2e/harness/errors.ts

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+// SPDX-License-Identifier: GPL-2.0-or-later
+// Copyright (C) 2026 Bryan Joshua Pedini
+
+/**
+ * Test-infrastructure errors.
+ *
+ * A HarnessError means "the harness could not obtain a trustworthy observation"
+ * (timeout, unexpected signal, hung process, spawn failure, leaked resource,
+ * missing prerequisite, unparseable protocol data). It is never a valid outcome
+ * for an expected-failure assertion: helpers throw it instead of returning a
+ * result, so a negative test can never pass because something crashed or hung.
+ */
+export type HarnessErrorKind =
+  | 'timeout' // a process / request / wait exceeded its deadline
+  | 'signal' // a process was terminated by a signal nobody asked for
+  | 'spawn' // a process could not be started
+  | 'hang' // a process did not exit after being asked to
+  | 'leak' // a resource (process, container, directory) outlived its owner
+  | 'prerequisite' // the environment is missing something the suite needs
+  | 'protocol' // a peer produced data the harness cannot parse
+  | 'transport' // an HTTP client failed below HTTP (refused, reset) where a response was required
+  | 'infrastructure'; // anything else that is the harness's fault, not the app's
+
+export class HarnessError extends Error {
+  readonly kind: HarnessErrorKind;
+  readonly details?: unknown;
+
+  constructor(kind: HarnessErrorKind, message: string, details?: unknown) {
+    super(`[harness:${kind}] ${message}`);
+    this.name = 'HarnessError';
+    this.kind = kind;
+    this.details = details;
+  }
+}
+
+export function isHarnessError(e: unknown): e is HarnessError {
+  return e instanceof HarnessError;
+}