Establish the pytest harness and migrate `press selftest` into reusable tests

[clintecker]

Problem

press selftest is a valuable installed-package smoke command, but its monolithic assertions do not provide isolated discovery, parametrization, fixtures, property tests, or reusable contract results. The accumulated-trust program needs a conventional test runner and shared support package before the higher test layers can be enforced.

Scope

Bootstrap the test harness without removing the portable press selftest command.

Acceptance criteria

• Add pytest and coverage tooling to the development dependencies with strict marker/config validation and importlib import mode.
• Establish tests/unit, tests/contracts, tests/damage, tests/graph, tests/integration, tests/e2e, and tests/support.
• Move or wrap current selftest assertions as independently selectable tests without duplicating their policy logic.
• Keep press selftest as an installed-wheel smoke command calling reusable lightweight contract functions that do not require pytest at runtime.
• Provide typed helpers for active return-value, exception, diagnostic, filesystem, and subprocess assertions.
• Isolate BOOK_ROOT, cwd, caches, environment variables, and generated files in every test.
• CI reports test, lint, typing, and integration failures as distinct gates.
• Document local commands for each layer and for the complete suite.

Program relationship

This is the foundation issue for native milestone v1.12 — Trust foundations. Later issues may depend on its directory layout and support APIs, but it must remain useful and mergeable on its own.