Resolve orphaned directories not in SUMMARY.md

Several directories exist in the tree but aren't wired into the table of contents (`SUMMARY.md`). Decide fold-in vs delete for each:

- [ ] `chapter-5-domain-specific-language/` — three prose files (`01-creating-our-own-langauge.md`, `02-a-simple-game.md`, `03-langauge-design.md`); looks like an earlier draft of the CardLang/Compiler material. Filename typo: "langauge".
- [ ] `chapter-4-crypto-cryptography/` — only a `sample/` dir (duplicate numbering of Ch 3)
- [ ] `chapter-pvm/` — only a `sample/` dir; referenced in README but not SUMMARY
- [ ] `src/` — `src/chap_5/node/README.md` is a 2-line stub; clarify relationship to `code/`

Tracked in TODO.md.

Start with SUMMARY.md, README, and TODO.md, then inspect the listed directories and the stub at src/chap_5/node/README.md. Compare their contents and references to determine the disposition of each directory. Done means every orphaned directory has a documented fold-in or deletion decision and the book references are consistent.

Written by the indexing model from the issue text.