RR-4 tsdown: remove tsup

Deletes tsup.config.ts and the tsup devDependency. The devDependency line
was removed with an editor followed by 'pnpm install' rather than
'pnpm remove', which re-resolves the whole graph against the registry and
takes minutes.

Dropping tsup also finally removes @microsoft/api-extractor and its whole
closure from the lockfile. api-extractor was deleted as a direct
devDependency back in 8058bb9 but survived because tsup declares it as an
optional peer dependency and pnpm kept the existing resolution rather than
pruning it. Lockfile goes from 841 to 670 package entries.

Author: markerikson

package.json

@@ -87,7 +87,6 @@
     "redux": "^5.0.1",
     "rimraf": "^6.1.3",
     "tsdown": "^0.22.14",
-    "tsup": "^8.5.1",
     "typescript": "^5.8.2",
     "unrun": "^0.3.1",
     "vitest": "^4.1.8"