chore(repo): bootstrap public book engine

[BenAlaa]

Outcome

Establishes the clean repository foundation for the source-grounded T3 Code architecture book without publishing chapter content yet.

Scope

• Add the Astro book engine, layouts, styles, shared figures, and interactive labs.
• Add the immutable source lock, generated excerpts, source manifests, and validation scripts.
• Add repository governance, licensing, contribution templates, and the pull-request quality workflow.
• Add the accessible SVG favicon and supplied book cover.

Non-goals

• Chapter content, tests, the public project guide, and the Pages deployment workflow arrive in the next reviewed publication PR.
• This bootstrap does not enable public visibility or Pages.

Evidence

• Canonical source lock: sources/t3code.lock.json
• Primary anchors:
  • src/layouts/BookLayout.astro
  • src/components/Mermaid.astro
  • scripts/validate-sources.mjs
  • .github/workflows/quality.yml
• Claim class notes: no narrative claims are published in this bootstrap.
• Discrepancies or uncertainty: none.

Visuals and interaction

The reusable book shell, diagram explorer, and labs preserve keyboard, reduced-motion, no-JavaScript, responsive, and print contracts. Their complete route-level validation follows with the edition content.

Validation

• Bootstrap diff reviewed for excluded chapter content and publication files
• Source lock and generated artifacts retained unchanged
• Conventional commit subject verified
• Replacement repository remains private

Review focus

Confirm that this PR contains only the reusable engine, evidence pipeline, and repository foundation; the complete edition must remain isolated to the next PR.