runtime-extension fixture

A tiny runtime-loadable ServerKit extension (plan 25 #12). It ships a prebuilt, externalized frontend/dist/index.mjs and no source/build tooling, so it doubles as:

a backend test fixture (test_runtime_frontend.py) — install it and assert the dist bundle survives a local install, its sha256 is recorded, and it shows up in the /plugins/contributions frontends map;
a render smoke test target for the screenshots skill (a real extension page that loads through the runtime path).

The bundle imports its shared libraries (react/jsx-runtime, serverkit-sdk) as bare specifiers — exactly what a vite build with those libs externalized emits — so the panel resolves them to its own singletons via the import map. It bundles no React of its own. Hand-checked-in on purpose: no npm install/build is needed to run the tests.