✨ Add knip for unused dependency and dead code detection

- Add check:knip using --include dependencies,exports,files
  (skips the noisy 'types' category, which produces false positives
  for libraries whose exported types are part of the public API)
- Remove tslib and type-fest (both caught as unused by knip)
- Add 'knip' to cspell word list
- No knip config file: the --include flag keeps the scope targeted
  without boilerplate, matching the 'keep it simple' principle
- Document in project-specs.md (CHECK section) and README.md

cspell.json

@@ -18,7 +18,8 @@
         "msvc",
         "publint",
         "attw",
-        "arethetypeswrong"
+        "arethetypeswrong",
+        "knip"
     ],
     "ignorePaths": ["dist", "node_modules", "coverage", "*.svg", ".gitignore"]
 }