End-to-end tests v3

[dhilt]

This MR replaces the legacy E2E engine with E2E v3: 31 specs and 503 tests running in real Chromium through Vitest Browser Mode and Playwright. The new infrastructure:

• imports TypeScript directly from src;
• uses an isolated TestHost with real DOM, datasource, and Workflow;
• provides shared lifecycle, expectations, datasource modes, and automatic DOM ↔ buffer invariants;
• uses observable workflow state instead of spies and artificial delays;
• removes the old fixtures, page objects, Playwright config, and browser globals;
• supports headless, UI/debug, and coverage modes.

CI now enforces separate unit/E2E coverage gates and publishes coverage reports and failure-only browser artifacts.

Additional regression tests fixed

• falsy datasource errors,
• Reactive.once(),
• pending Adapter promises on dispose,
• frequent-size tie handling,
• and boolean reactive resets.