chore: address maintainer review feedback (round 4)

- delete unused tokenizeOursSimple and the now-redundant
  tokenizer-vs-csstools.mjs (ours already delegates to
  @csstools/css-tokenizer, so the comparison was circular)
- dedupe corpus loading between benchmark.mjs and show-divergences.mjs
  into scripts/lib/corpus.mjs
- drop the type-coverage dependency; strict tsc already covers this and
  CI never ran the quality script anyway
- rename src/lib/type.js to convertUnits.js to stop it reading as a
  TS-types or CSS-<type> file
- document each script's purpose in scripts/README.md

Author: dkryaklin

package.json

@@ -24,9 +24,7 @@
   "scripts": {
     "lint": "eslint . && tsc",
     "test": "node --test",
-    "test:mutation": "stryker run",
-    "quality:type-coverage": "type-coverage --at-least 97.5",
-    "quality": "pnpm quality:type-coverage && pnpm lint"
+    "test:mutation": "stryker run"
   },
   "author": "Andy Jansson",
   "license": "MIT",
@@ -46,7 +44,6 @@
     "fast-check": "^4.7.0",
     "postcss": "^8.5.15",
     "prettier": "^3.8.4",
-    "type-coverage": "^2.29.7",
     "typescript": "~6.0.3"
   },
   "dependencies": {