Show how fast a recording not in real time goes as a speed

The tempo of a recording not in real time read as how many times
slower than real time it was made. "5.1x real time" for a recording
that took five times as long as its video looked like it ran five
times faster.

- The log line at the end and the notice now give seconds of video per
  second it took, with two decimals: 0.20x for the example above.
- It may go above 1x. Nothing holds a recording not in real time back
  to the wall clock, and a cheap scene on a fast screen makes more
  frames a second than the video has.
- A recording of a single frame took 0 s and has no tempo line now,
  instead of a division by zero.

Tests:
- tsc, eslint, Prettier clean
- Gate on melektaus-atx: green, 311 files, 4821 tests
- No check reads the figure itself. 68j looks for "x real time, and
  has every frame", which still matches.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/PlaySession.ts

@@ -322,7 +322,7 @@ export class PlaySession {
     if (steady && !this.state.paused)
       return (
         `Recording "${steady.name}" not in real time: ${steady.seconds.toFixed(1)} s of video, ` +
-        `${steady.slower.toFixed(1)}x real time · Esc stops`
+        `${steady.speed.toFixed(2)}x real time · Esc stops`
       );
     if (this.state.paused) {
       const why = this.breakAt === null ? 'Paused' : `Break in ${this.breakAt}`;