Fix stale/mismatched engine cache causing "ASM_CONSTS[...] is not a function" live

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.

public/sw.js

@@ -1,9 +1,16 @@
-// Bump this on every release that changes the app shell or the vendored
-// engine build — it's the only thing that invalidates old caches, since
+// Bump manually (via `pnpm sw:bump`) whenever SHELL_ASSETS or the shell's
+// caching behavior changes — nothing else invalidates the shell cache, since
 // none of the cached URLs below are content-hashed by us.
-const CACHE_VERSION = "v2";
-const SHELL_CACHE = `shell-${CACHE_VERSION}`;
-const ENGINE_CACHE = `engine-${CACHE_VERSION}`;
+const SHELL_VERSION = "v2";
+// Written automatically by scripts/copy-engine-assets.mjs from the installed
+// @valknar/vpinball-wasm version — do not edit by hand. Ties the engine
+// cache key to the actual engine build, so a version bump always busts
+// stale caches instead of relying on a human to remember a separate step
+// (which is exactly how returning visitors ended up on a stale, and
+// possibly internally inconsistent, engine bundle before this existed).
+const ENGINE_VERSION = "0.3.4";
+const SHELL_CACHE = `shell-${SHELL_VERSION}`;
+const ENGINE_CACHE = `engine-${ENGINE_VERSION}`;
 
 const SHELL_ASSETS = [
   "/",