v0.1.0-alpha.4

Alpha release with the optional Chromium fidelity engine.

Highlights:

• Add @vellora/engine-chromium as an opt-in Chromium-backed renderer.
• Add explicit reference-vs-subject fidelity checks to vellora doctor, including local Puppeteer PDF comparison support.
• Centralize fidelity policy parsing and include the Chromium engine package in the release pipeline.
• Remove internal docs/superpowers material from the public docs tree.