Write Chapter 8 (Writing a Compiler) body pages

The chapter currently has only an introduction page (`chapter-08-writing-a-compiler/01_introduction.md`). The intro promises one page per compiler stage; these need to be written:

- [ ] Lexing — source text into tokens
- [ ] Parsing — tokens into an AST
- [ ] Semantic analysis — type-checking, name resolution, validation
- [ ] Intermediate representation
- [ ] Code generation — bytecode / target language / interpreter input
- [ ] Runtime / VM

Worked examples should be drawn from CardLang (Chapter 7).

Tracked in TODO.md.

Start with chapter-08-writing-a-compiler/01_introduction.md and review the CardLang material from Chapter 7 for worked examples. Write the six promised pages covering each listed compiler stage, then update TODO.md and confirm the chapter has a complete, coherent explanation with examples.

Written by the indexing model from the issue text.