The service worker's engine cache key relied on a manual CACHE_VERSION
bump that was forgotten across the last two @valknar/vpinball-wasm
version bumps, so returning visitors kept getting served old cached
vpinball.js/.wasm/.data via cache-first with no revalidation — the origin
server itself was serving a correct, matching 0.3.4 set the whole time
(verified via headers/sizes against the local build).
Split CACHE_VERSION into a manually-bumped SHELL_VERSION and an
ENGINE_VERSION that copy-engine-assets.mjs now writes automatically from
the installed engine's package.json on every build, so the engine cache
always busts in lockstep with the dependency version — no step to forget.