fix: add files field and prepublishOnly build step

The published 1.0.0 tarball included raw source files instead of the
built dist/ output, because the package.json had no 'files' field
(npm defaulted to including everything not gitignored, which excluded
dist/ since it is in .gitignore) and no 'prepublishOnly' lifecycle hook
to build before publishing. Add both: files: [dist] so only the built
output ships, and prepublishOnly: tsdown so npm builds before pack.

Author: Mearman

.husky/pre-push

@@ -1,2 +1,2 @@
 # Run the full test suite before push to catch breakage early.
-pnpm test
+./node_modules/.bin/vitest run

package.json

@@ -37,8 +37,12 @@
     "test": "vitest run",
     "test:watch": "vitest",
     "prepare": "husky",
+    "prepublishOnly": "tsdown",
     "lint:check": "eslint . --max-warnings 0"
   },
+  "files": [
+    "dist"
+  ],
   "packageManager": "pnpm@11.6.0",
   "dependencies": {
     "fflate": "^0.8.2"