build: migrate from tsup to tsdown

Tsdown simplifies build processes with a zero-config bundling and better integration with modern tooling.

Changes:
- Replace `tsup` with `tsdown`
- Remove @arethetypeswrong/cli (attw) - no longer needed
- Remove attw validation step from CI workflow

Author: tim-hm

.github/workflows/ci.yaml

@@ -21,7 +21,6 @@ jobs:
       - run: pnpm build
       - run: pnpm exec biome ci
       - run: tsc
-      - run: pnpm attw
 
   test:
     needs: check

package.json

@@ -9,8 +9,8 @@
   "type": "module",
   "exports": {
     ".": {
-      "types": "./dist/lib.d.ts",
-      "import": "./dist/lib.js"
+      "types": "./dist/lib.d.mts",
+      "import": "./dist/lib.mjs"
     },
     "./package.json": "./package.json"
   },
@@ -20,8 +20,7 @@
     "install-hooks": "lefthook install",
     "test": "vitest --run",
     "build:proto": "npx buf generate",
-    "build": "pnpm build:proto && tsup",
-    "attw": "attw --profile esm-only --pack ."
+    "build": "pnpm build:proto && tsdown"
   },
   "peerDependencies": {
     "pino": "^9.7.0"
@@ -41,7 +40,6 @@
     "zod": "^4.1.12"
   },
   "devDependencies": {
-    "@arethetypeswrong/cli": "^0.18.2",
     "@biomejs/biome": "2.3.5",
     "@bufbuild/buf": "^1.59.0",
     "@bufbuild/protoc-gen-es": "^2.10.1",
@@ -53,12 +51,13 @@
     "@types/node": "^24.10.1",
     "@types/semver": "^7.7.1",
     "@vitest/coverage-v8": "^4.0.9",
+    "abitype": "^1.1.1",
     "docker-compose": "^1.3.0",
     "dotenv": "^17.2.3",
     "fast-check": "^4.3.0",
     "lefthook": "^2.0.4",
     "semver": "^7.7.3",
-    "tsup": "^8.5.1",
+    "tsdown": "^0.16.4",
     "tsx": "^4.20.6",
     "type-fest": "^5.2.0",
     "typedoc": "^0.28.14",
@@ -74,5 +73,14 @@
     "README.md",
     "DOCUMENTATION.md",
     "CONTRIBUTING.md"
-  ]
+  ],
+  "tsdown": {
+    "entry": "./src/lib.ts",
+    "format": "esm",
+    "clean": true,
+    "alias": {
+      "#": "./src",
+      "#tests": "./tests"
+    }
+  }
 }