Echo/how does echo work book

[flyingrobots]

Title: Echo/how does echo work book

Summary

• Experiments with the idea of switching all docs over to LaTeX and PDFs generated by CI...
• Removes old "rollup" echo-total.md massive markdown file
• Adds a CI job to validate SPDX headings
• Adds a repair mode to the SPDX headings script
• Fixes a ton of busted SPDX headings

Scope

• Tests
• Docs
• Runtime code

Checklist

• Docs Guard satisfied (updated docs/execution-plan.md and docs/decision-log.md when non-doc files changed)
• CI green (fmt, clippy, tests, rustdoc, audit/deny)
• Kept PR small and focused (one thing)