test: add comprehensive unit tests for lib modules

Add 163 unit tests covering all core library utilities:

- api-docs.test.ts: Type formatting, parameter formatting, JSDoc
  extraction, filtering, grouping (49 tests)
- markdown.test.ts: Markdown parsing, slugify, JSDoc links,
  TOC extraction (21 tests)
- type-references.test.ts: Recursive type reference extraction
  from all TypeScript definition types (26 tests)
- signature-formatters.test.ts: TypeScript signature formatting
  for functions, methods, classes, interfaces (25 tests)
- api-markdown.test.ts: API documentation Markdown generation,
  cross-package linking, Related Links section (18 tests)

Remove obsolete tests/links_test.ts in favor of new test structure.

Author: lambdalisue