Only touch the DOM when a displayed value changes

The readout hung and then skipped numbers in a focused desktop window.
render() runs every animation frame and was writing unconditionally —
measured at 11 DOM mutations per frame — whether or not anything had
changed. Two of those are the .time text node, a glyph run up to 190px, so
every frame forced a style recalc and a full relayout to redraw digits that
turn over once a second. Routing the per-frame writes through change guards
takes a steady frame from 11 writes to 0.

The timing itself was never at fault and is untouched here. elapsed() is one
subtraction from a single monotonic performance.now() stamp and the reserve
is debited once, in tap(), so nothing accumulates per frame and a dropped
frame cannot cost or gift anyone a millisecond. Measured against wall clock
over a running turn: 0.0000ms drift across 5s, 12s and 17s segments.

Also fixes a real skip in fmt(). It switched from m:ss to tenths at
ms < 10000 with both branches rounding up, so the coarse branch read a
second high: counting down gave 0:11 for a full second, 0:10 for a single
millisecond, then 10.0. Handing 10000ms to the tenths branch gives a
monotone 0:11 -> 10.0 -> 9.9.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

public_html/sw.js

@@ -1,5 +1,5 @@
 /* Cache-first: once installed the clock never touches the network again. */
-const CACHE = "bgclock-v6";
+const CACHE = "bgclock-v7";
 const FILES = [
   "./",
   "./index.html",