build(deps-dev): bump tsdown from 0.15.12 to 0.16.6

Bumps [tsdown](https://github.com/rolldown/tsdown) from 0.15.12 to 0.16.6.
- [Release notes](https://github.com/rolldown/tsdown/releases)
- [Commits](rolldown/tsdown@v0.15.12...v0.16.6)

---
updated-dependencies:
- dependency-name: tsdown
  dependency-version: 0.16.6
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package.json

@@ -29,7 +29,7 @@
     "rollup": "^4.48.0",
     "rollup-plugin-dts": "^6.2.3",
     "serpack": "^0.1.34",
-    "tsdown": "^0.15.9",
+    "tsdown": "^0.16.8",
     "typescript": "^5.6.2",
     "typescript-eslint": "^8.46.2",
     "vite": "^7.1.6",