Introduces showcase/shell/src/lib/runtime-config.ts (server-only —
imports next/cache and is read at request time by the root layout)
plus runtime-config.client.ts (reads window.__SHOWCASE_CONFIG__
injected by the layout). Shell's RuntimeConfig contains baseUrl and
posthogHost. getRuntimeConfigEdge() provides the Edge-runtime variant
for middleware (skips unstable_noStore).
Adds vitest config + dev deps to package.json and red-green tests for
both modules. Tests verify env-vs-fallback precedence, trailing-slash
stripping, no-module-load-freeze (live process.env reads per call),
and the Edge wrapper's noStore-skip behavior.
Refs plan-B §B7.