chore: pin toolchain and add type checking

- Pin TypeScript as a direct dependency. It was only present transitively
  via typescript-eslint, so nothing enforced tsconfig.json and nine
  latent type errors had gone unnoticed.
- Add `pnpm typecheck` (`tsc --noEmit`) and document it.
- Add .npmrc with engine-strict, so the declared Node and pnpm ranges
  are actually enforced.
- Ignore doc_build/ in ESLint. It was unignored and reported 1650 errors
  from build output, which made lint unusable as a gate.
- Refresh README, CLAUDE.md, AGENTS.md and CONTRIBUTING.md to match the
  current structure, build targets and navigation rules.

eslint.config.mjs

@@ -1,10 +1,10 @@
-import js from '@eslint/js';
-import globals from 'globals';
-import ts from 'typescript-eslint';
+import js from "@eslint/js"
+import globals from "globals"
+import ts from "typescript-eslint"
 
 export default [
   { languageOptions: { globals: globals.browser } },
   js.configs.recommended,
   ...ts.configs.recommended,
-  { ignores: ['dist/'] },
-];
+  { ignores: ["dist/", "doc_build/"] },
+]