Make course material self-contained

[Doppp]

Summary

• make lesson Markdown the canonical, self-contained teaching source across all thirteen AI foundations lessons
• add prerequisite bridges, worked examples, implementation connections, failure modes, and explanation-based recaps
• bump AI from First Principles to 0.4.0-foundations.1 and update authoring and tutor policy
• correct gradient trend, Model Atlas disclosure and trace semantics, and stable loss coverage
• add rendered-content browser coverage and keep all course prose within supported CommonMark

Validation

• pnpm check
• pnpm build
• pnpm course:test
• pnpm test:browser
• pnpm site:test

All checks passed in a clean detached checkout under Node 24 and pnpm 11.

Progress compatibility

The course version bump intentionally creates a new browser-progress namespace. Existing 0.3 progress remains stored, but it does not automatically resume under 0.4.