feat(sw): offer a reload when a newer version is ready

The shell is served cache-first, which is what makes the app work on a
train and also what makes a deploy invisible: a reader with the tab open
— the reader this app is built for — keeps running the bundle they first
loaded, so a new game or a corrected date reaches their device and sits
there with nothing saying why the page looks unchanged. An old app shown
as current is the same failure as old events shown as current.

So the worker now installs quietly and waits instead of calling
skipWaiting(), the page notices it waiting and says so, and the reader's
tap sends the skip-waiting message and reloads on controllerchange. The
app never reloads itself: someone may be mid-way through typing in one of
their own events, and the notice says what a reload costs (their place on
the page) and what it does not (marks and notes live in localStorage).

Detection is derived rather than remembered. build:static grew into a
script that stamps sw.js with a hash of the built shell, because the
browser only offers a worker whose bytes differ, and the predecessor —
a hand-bumped CACHE_VERSION — had already been forgotten once. The feed
is deliberately not part of that hash: it changes twice a day, needs no
reload, and announcing it would teach readers to dismiss the notice
unread. The cache name stays put for the same reason a per-build one
would be wrong — it holds the feed an offline reader is reading.

A first install is not an update and stays silent.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

docs/PRD.md

@@ -20,7 +20,9 @@ A single-page web app that answers three questions:
 - Not an account system. There is no login, no profile, no cloud sync.
 - Not a wiki. It does not explain how to complete an event, only that it exists and when it ends.
 - Not a notification service. No push, no email, no background alerts. (A browser-local reminder
-  is a plausible v2; it is out of scope for v1.)
+  is a plausible v2; it is out of scope for v1.) F14 is not an exception to this: it is the open page
+  disclosing something about *itself*, in the tab, while the reader is looking at it — nothing is
+  delivered anywhere, and the app is never told to wake anybody up.
 - Not a damage calculator, build planner, or pull tracker.
 
 ## Users
@@ -163,6 +165,25 @@ The sources that compile these calendars, and the studios that make the games, a
 screen as the data rather than one navigation step away. The page states plainly that it is
 unofficial and unaffiliated, and that the source page is the authority when the two disagree.
 
+**F14 — Say when a new version of the app is ready.**
+F10 caches the shell so the app survives losing signal, and the same cache is why a reader who never
+closes the tab keeps running the version they first loaded. A new game, a repaired parser or a
+corrected date then reaches their device and sits there unused, with the page looking unchanged and
+nothing saying why. **Presenting an old app as current is the same failure as presenting old events
+as current** (F7), so it is disclosed the same way: a notice on any screen, with one action that
+reloads into the new version.
+
+It is an offer, not a swap. The app never reloads itself — doing so mid-sentence while someone types
+in their own event (F13) would cost them work to save the app a tap. It says what a reload costs
+(their place on the page) and what it does not (everything they have marked, typed or ticked lives in
+`localStorage`, not in the bundle). Dismissing is free and the offer returns on the next load, which
+is also why it need not nag.
+
+A first install is not an update and is not announced — nothing is being replaced, and telling a
+first-time reader a new version is available would be false. Neither is a feed refresh: new events
+arrive without a reload, and calling that a new version would train readers to dismiss the notice
+unread.
+
 **F7 — Freshness disclosure.**
 The footer shows when the feed was last updated, per game. If a game's data is more than 48 hours
 stale, its lane carries a warning badge. Never present stale data as current — the whole value