When a navigation fetch failed and the cache had no entry for the exact
URL (e.g. '/' while only 'index.html' is precached, common while the
local server restarts), the fetch handler resolved respondWith() to
undefined, producing 'TypeError: Failed to convert value to Response'.
The navigate fallback now chains caches.match(request) -> cached
index.html -> an explicit 503 offline Response. sw.js requests are
network-only with the same offline Response (previously they fell into
the navigate branch), and a new unit test loads sw.js in a VM sandbox
covering navigate/cache/asset/api paths. SW cache v97 -> v98.
