Create docs/developers-guide.md documenting refactored parser module structure

[coderabbitai]

Background

PR #259 refactored several internal parser modules below the 400-line maintainability guideline, introducing new modules with distinct responsibilities:

• src/parser/ast/expr/sexpr.rs — S-expression formatting for Expr
• src/parser/ast/rule/classification.rs — rule-body term classification
• src/parser/expression/pratt/delay.rs — delay-postfix parsing
• src/parser/expression/pratt/diff.rs — diff-marker postfix helpers
• src/parser/expression/pratt/postfix.rs — postfix-operator dispatch
• src/parser/tests/expression/fixtures/ — structured test fixture hierarchy

No developer guide documents these modules, their responsibilities, or the architectural boundaries between them.

Required work

Create docs/developers-guide.md covering:

1. The overall parser module layout and the 400-line size guideline.
2. Each new module: its responsibility, what it owns, and what it delegates.
3. Architectural boundaries (e.g. why sexpr formatting is separate from the AST definition; why classification is separate from rule parsing).
4. How to navigate the test fixture hierarchy when adding new expression parser tests.

References

• PR Refactor large parser modules into smaller components under 400 lines (#223) #259: Refactor large parser modules into smaller components under 400 lines (#223) #259
• Raised as out-of-scope in review comment: Refactor large parser modules into smaller components under 400 lines (#223) #259
• Requested by @leynos